diff --git a/lib/src/git_lister.dart b/lib/src/git_lister.dart
new file mode 100644
index 0000000..6b20cdc
--- /dev/null
+++ b/lib/src/git_lister.dart
@@ -0,0 +1,45 @@
+// Copyright 2026 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io';
+
+import 'github_client.dart' show ProcessRunner;
+
+/// Signature for querying RFC files on a remote/base git branch.
+typedef GitListFunction =
+    Future<Set<String>> Function({String baseBranch, String rfcDir});
+
+/// Default implementation querying git via `git ls-tree`.
+Future<Set<String>> defaultGitList({
+  String baseBranch = 'origin/main',
+  String rfcDir = 'rfc',
+  ProcessRunner processRunner = Process.run,
+}) async {
+  try {
+    final result = await processRunner('git', [
+      'ls-tree',
+      '-r',
+      '--name-only',
+      baseBranch,
+      '--',
+      '$rfcDir/',
+    ]);
+    if (result.exitCode != 0) {
+      stdout.writeln('exit code: ${result.exitCode}');
+      stdout.writeln('git ls-tree stdout:');
+      stdout.writeln(result.stdout);
+      stderr.writeln('git ls-tree stderr:');
+      stderr.writeln(result.stderr);
+      return const <String>{};
+    }
+    final stdoutStr = result.stdout as String;
+    return stdoutStr
+        .split('\n')
+        .map((s) => s.trim())
+        .where((s) => s.isNotEmpty)
+        .toSet();
+  } catch (_) {
+    return const <String>{};
+  }
+}
diff --git a/lib/src/github_client.dart b/lib/src/github_client.dart
new file mode 100644
index 0000000..182756f
--- /dev/null
+++ b/lib/src/github_client.dart
@@ -0,0 +1,57 @@
+// Copyright 2026 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io';
+
+/// Signature for running an external process asynchronously.
+typedef ProcessRunner =
+    Future<ProcessResult> Function(String executable, List<String> arguments);
+
+/// Abstract client for interacting with the GitHub API / CLI.
+abstract interface class GitHubClient {
+  /// Checks whether a GitHub user exists.
+  Future<bool> userExists(String username);
+}
+
+/// Production implementation using the `gh` CLI.
+class CliGitHubClient implements GitHubClient {
+  /// The process runner used to execute external commands.
+  final ProcessRunner processRunner;
+
+  const CliGitHubClient({this.processRunner = Process.run});
+
+  @override
+  Future<bool> userExists(String username) async {
+    try {
+      final result = await processRunner('gh', [
+        'api',
+        'users/$username',
+        '--silent',
+      ]);
+      if (result.exitCode != 0) {
+        stdout.writeln('exit code: ${result.exitCode}');
+        stdout.writeln('gh api stdout:');
+        stdout.writeln(result.stdout);
+        stderr.writeln('gh api stderr:');
+        stderr.writeln(result.stderr);
+      }
+      return result.exitCode == 0;
+    } catch (_) {
+      return false;
+    }
+  }
+}
+
+/// Test double with in-memory state for hermetic unit testing.
+class FakeGitHubClient implements GitHubClient {
+  final Set<String> existingUsers;
+
+  FakeGitHubClient({Set<String>? existingUsers})
+    : existingUsers = existingUsers ?? <String>{};
+
+  @override
+  Future<bool> userExists(String username) async {
+    return existingUsers.contains(username);
+  }
+}
diff --git a/lib/src/taxonomy.dart b/lib/src/taxonomy.dart
new file mode 100644
index 0000000..92963b0
--- /dev/null
+++ b/lib/src/taxonomy.dart
@@ -0,0 +1,72 @@
+// Copyright 2026 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:file/file.dart';
+
+/// Represents the dynamic taxonomy of Flutter subsystems extracted from RFC 000.0001.
+class Taxonomy {
+  /// Set of all valid 3-digit category strings (e.g. "000", "010", "110", "210").
+  final Set<String> categories;
+
+  /// Optional mapping from category string to human-readable title.
+  final Map<String, String> categoryNames;
+
+  const Taxonomy(this.categories, [this.categoryNames = const {}]);
+
+  /// Checks whether [category] is a recognized subsystem classification.
+  bool isValidCategory(String category) => categories.contains(category);
+
+  /// Parses taxonomy categories from markdown content of RFC 000.0001.
+  static Taxonomy fromMarkdown(String markdown) {
+    final categories = <String>{};
+    final categoryNames = <String, String>{};
+
+    // Match section headers: ### 000 – General, Process, & Meta
+    final sectionHeaderRegex = RegExp(
+      r'^###\s+([0-9]{3})\s+[–—-]\s*(.*)$',
+      multiLine: true,
+    );
+    for (final match in sectionHeaderRegex.allMatches(markdown)) {
+      final code = match.group(1)!;
+      final name = match.group(2)?.trim() ?? '';
+      categories.add(code);
+      categoryNames[code] = name;
+    }
+
+    // Match list items: * **110:** Foundation & Low-level
+    final listItemRegex = RegExp(
+      r'^\*\s+\*\*([0-9]{3}):?\*\*:?\s*(.*)$',
+      multiLine: true,
+    );
+    for (final match in listItemRegex.allMatches(markdown)) {
+      final code = match.group(1)!;
+      final name = match.group(2)?.trim() ?? '';
+      categories.add(code);
+      if (name.isNotEmpty) {
+        categoryNames[code] = name;
+      }
+    }
+
+    return Taxonomy(categories, categoryNames);
+  }
+
+  /// Loads the taxonomy from RFC 000.0001 on the given [fs].
+  static Future<Taxonomy> load(FileSystem fs) async {
+    File? file;
+
+    const defaultPath =
+        'rfc/000.0001-flutter-architecture-and-reference-taxonomy.md';
+    file = fs.file(defaultPath);
+
+    if (!await file.exists()) {
+      throw StateError(
+        'Could not locate RFC 000.0001 taxonomy document in "rfc". '
+        'Ensure rfc/000.0001-flutter-architecture-and-reference-taxonomy.md exists.',
+      );
+    }
+
+    final content = await file.readAsString();
+    return Taxonomy.fromMarkdown(content);
+  }
+}
diff --git a/test/git_lister_test.dart b/test/git_lister_test.dart
new file mode 100644
index 0000000..6bf5b33
--- /dev/null
+++ b/test/git_lister_test.dart
@@ -0,0 +1,61 @@
+// Copyright 2026 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:rfc_tools/src/git_lister.dart';
+import 'package:test/test.dart';
+
+import 'mock_process_runner.dart';
+
+void main() {
+  group('defaultGitList', () {
+    test('returns parsed file set on zero exit code', () async {
+      final runner = MockProcessRunner(
+        exitCode: 0,
+        stdout: 'rfc/000.0001-taxonomy.md\nrfc/000.0002-process.md\n',
+      );
+
+      final files = await defaultGitList(
+        baseBranch: 'origin/main',
+        processRunner: runner.run,
+      );
+
+      expect(
+        files,
+        equals({'rfc/000.0001-taxonomy.md', 'rfc/000.0002-process.md'}),
+      );
+      expect(runner.calls, hasLength(1));
+      expect(
+        runner.calls.first.arguments,
+        equals(['ls-tree', '-r', '--name-only', 'origin/main', '--', 'rfc/']),
+      );
+    });
+
+    test('returns empty set on non-zero exit code', () async {
+      final runner = MockProcessRunner(
+        exitCode: 128,
+        stderr: 'fatal: not a valid object name',
+      );
+
+      final files = await defaultGitList(
+        baseBranch: 'origin/main',
+        processRunner: runner.run,
+      );
+
+      expect(files, isEmpty);
+    });
+
+    test('returns empty set when process throws', () async {
+      final runner = MockProcessRunner(
+        exceptionToThrow: Exception('process failed'),
+      );
+
+      final files = await defaultGitList(
+        baseBranch: 'origin/main',
+        processRunner: runner.run,
+      );
+
+      expect(files, isEmpty);
+    });
+  });
+}
diff --git a/test/github_client_test.dart b/test/github_client_test.dart
new file mode 100644
index 0000000..aad2b04
--- /dev/null
+++ b/test/github_client_test.dart
@@ -0,0 +1,69 @@
+// Copyright 2026 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io';
+import 'package:rfc_tools/src/github_client.dart';
+import 'package:test/test.dart';
+import 'mock_process_runner.dart';
+
+void main() {
+  group('CliGitHubClient', () {
+    test('default constructor uses Process.run', () {
+      const client = CliGitHubClient();
+      expect(client.processRunner, equals(Process.run));
+    });
+
+    group('userExists', () {
+      test('returns true when exitCode is 0', () async {
+        final runner = MockProcessRunner(exitCode: 0);
+        final client = CliGitHubClient(processRunner: runner.run);
+
+        final exists = await client.userExists('octocat');
+
+        expect(exists, isTrue);
+        expect(runner.calls, hasLength(1));
+        expect(runner.calls.first.executable, equals('gh'));
+        expect(
+          runner.calls.first.arguments,
+          equals(['api', 'users/octocat', '--silent']),
+        );
+      });
+
+      test('returns false when exitCode is non-zero', () async {
+        final runner = MockProcessRunner(exitCode: 1);
+        final client = CliGitHubClient(processRunner: runner.run);
+
+        final exists = await client.userExists('unknown-user');
+
+        expect(exists, isFalse);
+        expect(runner.calls, hasLength(1));
+        expect(runner.calls.first.executable, equals('gh'));
+        expect(
+          runner.calls.first.arguments,
+          equals(['api', 'users/unknown-user', '--silent']),
+        );
+      });
+
+      test('returns false when process runner throws', () async {
+        final runner = MockProcessRunner(
+          exceptionToThrow: const SocketException('network down'),
+        );
+        final client = CliGitHubClient(processRunner: runner.run);
+
+        final exists = await client.userExists('octocat');
+
+        expect(exists, isFalse);
+      });
+    });
+  });
+
+  group('FakeGitHubClient', () {
+    test('verifies user existence against existingUsers set', () async {
+      final client = FakeGitHubClient(existingUsers: {'user1'});
+
+      expect(await client.userExists('user1'), isTrue);
+      expect(await client.userExists('unknown'), isFalse);
+    });
+  });
+}
diff --git a/test/mock_process_runner.dart b/test/mock_process_runner.dart
new file mode 100644
index 0000000..132b52d
--- /dev/null
+++ b/test/mock_process_runner.dart
@@ -0,0 +1,44 @@
+// Copyright 2026 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io';
+
+typedef MockProcessHandler =
+    Future<ProcessResult> Function(String executable, List<String> arguments);
+
+/// In-memory mock process runner to record external process invocations and
+/// control process outputs hermetically in tests.
+class MockProcessRunner {
+  final List<({String executable, List<String> arguments})> calls = [];
+  MockProcessHandler? handler;
+  int exitCode;
+  dynamic stdout;
+  dynamic stderr;
+  Object? exceptionToThrow;
+
+  MockProcessRunner({
+    this.exitCode = 0,
+    this.stdout = '',
+    this.stderr = '',
+    this.exceptionToThrow,
+    this.handler,
+  });
+
+  Future<ProcessResult> run(String executable, List<String> arguments) async {
+    calls.add((
+      executable: executable,
+      arguments: List.unmodifiable(arguments),
+    ));
+    if (exceptionToThrow != null) {
+      throw exceptionToThrow!;
+    }
+    if (handler != null) {
+      return await handler!(executable, arguments);
+    }
+    return ProcessResult(1234, exitCode, stdout, stderr);
+  }
+
+  Future<ProcessResult> call(String executable, List<String> arguments) =>
+      run(executable, arguments);
+}
diff --git a/test/taxonomy_test.dart b/test/taxonomy_test.dart
new file mode 100644
index 0000000..fdff9b5
--- /dev/null
+++ b/test/taxonomy_test.dart
@@ -0,0 +1,64 @@
+// Copyright 2026 The Flutter Authors.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:file/memory.dart';
+import 'package:rfc_tools/src/taxonomy.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Taxonomy', () {
+    test('parses section headers and list items from markdown', () {
+      const sample = '''
+# RFC 000.0001: Architecture Taxonomy
+
+### 000 – General, Process, & Meta
+Governance and how the Flutter project itself functions.
+
+* **000:** RFC Process & Templates
+* **010:** Governance & Steering Committees
+
+### 100 – Flutter Framework Core
+The Dart-side architecture of Flutter.
+
+* **110:** Foundation & Low-level
+* **120:** Rendering Layer
+''';
+
+      final taxonomy = Taxonomy.fromMarkdown(sample);
+      expect(
+        taxonomy.categories,
+        containsAll(['000', '010', '100', '110', '120']),
+      );
+      expect(taxonomy.isValidCategory('000'), isTrue);
+      expect(taxonomy.isValidCategory('010'), isTrue);
+      expect(taxonomy.isValidCategory('110'), isTrue);
+      expect(taxonomy.isValidCategory('999'), isFalse);
+      expect(taxonomy.isValidCategory('abc'), isFalse);
+    });
+
+    test('loads successfully from FileSystem', () async {
+      final fs = MemoryFileSystem();
+      final file = fs.file(
+        'rfc/000.0001-flutter-architecture-and-reference-taxonomy.md',
+      );
+      await file.create(recursive: true);
+      await file.writeAsString('''
+# RFC 000.0001: Taxonomy
+### 100 – Core
+* **110:** Foundation
+''');
+
+      final taxonomy = await Taxonomy.load(fs);
+      expect(taxonomy.isValidCategory('110'), isTrue);
+      expect(taxonomy.isValidCategory('999'), isFalse);
+    });
+
+    test('throws StateError when taxonomy document does not exist', () async {
+      final fs = MemoryFileSystem();
+      await fs.directory('rfc').create(recursive: true);
+
+      expect(() => Taxonomy.load(fs), throwsStateError);
+    });
+  });
+}