feat(runway): Add CommitMessageResolver interface for squash and merge commit metadata

runway/extension/merger/BUILD.bazel

@@ -2,7 +2,10 @@ load("@rules_go//go:def.bzl", "go_library")
 
 go_library(
     name = "go_default_library",
-    srcs = ["merger.go"],
+    srcs = [
+        "commit_message.go",
+        "merger.go",
+    ],
     importpath = "github.com/uber/submitqueue/runway/extension/merger",
     visibility = ["//visibility:public"],
     deps = ["//api/runway/messagequeue:go_default_library"],

runway/extension/merger/commit_message.go

@@ -0,0 +1,38 @@
+// Copyright (c) 2026 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package merger
+
+//go:generate mockgen -source=commit_message.go -destination=mock/commit_message_mock.go -package=mock
+
+import "context"
+
+// CommitMessage is the message and authorship recorded on a squash or merge
+// commit.
+type CommitMessage struct {
+	// Message is the full commit message.
+	Message string
+	// AuthorName is the display name of the commit author.
+	AuthorName string
+	// AuthorEmail is the email address of the commit author.
+	AuthorEmail string
+}
+
+// CommitMessageResolver resolves the commit message and authorship for the
+// change named by a change URI. Implementations must be safe for concurrent
+// use.
+type CommitMessageResolver interface {
+	// Resolve returns the commit message for the change identified by uri.
+	Resolve(ctx context.Context, uri string) (CommitMessage, error)
+}

runway/extension/merger/mock/BUILD.bazel

@@ -2,7 +2,10 @@ load("@rules_go//go:def.bzl", "go_library")
 
 go_library(
     name = "go_default_library",
-    srcs = ["merger_mock.go"],
+    srcs = [
+        "commit_message_mock.go",
+        "merger_mock.go",
+    ],
     importpath = "github.com/uber/submitqueue/runway/extension/merger/mock",
     visibility = ["//visibility:public"],
     deps = [