Add Co-authored-by trailers to Merge's squash commit

api/openapi.yaml

@@ -173,6 +173,25 @@ paths:
         "404": { $ref: "#/components/responses/Error" }
         "409": { $ref: "#/components/responses/Error" }
 
+  /api/tasks/{taskId}/co-authors:
+    parameters:
+      - { name: taskId, in: path, required: true, schema: { type: string } }
+    get:
+      operationId: getCoAuthors
+      description: >
+        The humans other than the caller with a message in the task's transcript (sent its agent a message, or had
+        an inbox answer delivered under their name), in order of their first message. A merge by the caller gives each one with a git identity a
+        Co-authored-by trailer.
+      responses:
+        "200":
+          description: The co-authors.
+          content:
+            application/json:
+              schema:
+                type: array
+                items: { $ref: "#/components/schemas/Participant" }
+        "404": { $ref: "#/components/responses/Error" }
+
   /api/tasks/{taskId}/merge:
     parameters:
       - { name: taskId, in: path, required: true, schema: { type: string } }
@@ -180,9 +199,10 @@ paths:
       operationId: mergeTask
       description: >
         Starts the same agent turn as merge-in. When that turn ends normally, byre squashes the task's worktree
-        into one commit titled with the task's merge_title on the project's default branch, fast-forwards the project's
-        checkout to it, then removes the worktree and branch and marks the task merged. Nothing is pushed. Refused while
-        a turn is running or queued.
+        into one commit titled with the task's merge_title on the project's default branch, authored by the caller with
+        a Co-authored-by trailer for each of the task's co-authors, pushes it to origin, fast-forwards the project's
+        checkout to it, then removes the worktree and branch and marks the task merged. Refused while a turn is running
+        or queued.
       responses:
         "202":
           description: The merge turn was started or queued.
@@ -634,6 +654,15 @@ components:
         deletions: { type: integer }
         uncommitted: { type: integer, description: Paths with uncommitted changes in the worktree. }
 
+    Participant:
+      type: object
+      required: [login, display_name, git_name, git_email]
+      properties:
+        login: { type: string }
+        display_name: { type: string }
+        git_name: { type: string, description: Empty when the user hasn't set a git identity; they then get no trailer. }
+        git_email: { type: string }
+
     TaskCreate:
       type: object
       required: [title]