feat(rfc_tools): implement sequential RFC number assigner CLI

bin/assign_rfc_number.dart

@@ -0,0 +1,85 @@
+// 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:args/args.dart';
+import 'package:file/local.dart';
+import 'package:rfc_tools/src/assigner.dart';
+
+void main(List<String> arguments) async {
+  final parser = ArgParser()
+    ..addOption(
+      'target-file',
+      help:
+          'Specific RFC file path to assign (defaults to auto-detecting draft .0000).',
+    )
+    ..addFlag(
+      'dry-run',
+      negatable: false,
+      help: 'Simulate assignment without modifying files.',
+    )
+    ..addFlag(
+      'help',
+      abbr: 'h',
+      negatable: false,
+      help: 'Show usage instructions.',
+    );
+
+  ArgResults results;
+  try {
+    results = parser.parse(arguments);
+  } catch (e) {
+    stderr.writeln('Error parsing arguments: $e\n');
+    stderr.writeln(parser.usage);
+    exitCode = 1;
+    return;
+  }
+
+  if (results.flag('help')) {
+    stdout.writeln('RFC Number Assigner - Flutter RFC Repository Tooling\n');
+    stdout.writeln(parser.usage);
+    return;
+  }
+
+  final targetFile = results.rest.firstOrNull ?? results.option('target-file');
+  final dryRun = results.flag('dry-run');
+
+  const fs = LocalFileSystem();
+  final assigner = RfcAssigner(fs: fs);
+
+  try {
+    stdout.writeln('Assigning RFC number...');
+    final result = await assigner.assign(
+      targetPath: targetFile,
+      dryRun: dryRun,
+    );
+
+    if (result.dryRun) {
+      stdout.writeln(
+        '[DRY RUN] Would assign RFC identifier: ${result.newRfcId}',
+      );
+      stdout.writeln('[DRY RUN] Target file: ${result.oldPath}');
+      stdout.writeln('[DRY RUN] Rename to:   ${result.newPath}');
+    } else {
+      stdout.writeln(
+        'Successfully assigned RFC identifier: ${result.newRfcId}',
+      );
+      stdout.writeln('File: ${result.newPath}');
+
+      final githubOutput = Platform.environment['GITHUB_OUTPUT'];
+      if (githubOutput != null && githubOutput.isNotEmpty) {
+        await fs
+            .file(githubOutput)
+            .writeAsString(
+              'rfc_id=${result.newRfcId}\nnew_path=${result.newPath}\n',
+              mode: FileMode.append,
+            );
+      }
+    }
+  } catch (e) {
+    stderr.writeln('Assignment failed: $e');
+    exitCode = 1;
+    return;
+  }
+}