fix(git): pass actor_agent_id to pr_merge for workspace resolution

pr_merge falls back to task.assigned_to for workspace resolution, but
that field is None at merge time (submit_qa/pass_qa cleared it during
prior transitions). When project.workspace_path is unset, the resolver
raises ValidationError 'no workspace configured and no agent_id
provided' — surfaces as 500 from cell_pm_complete.

Add actor_agent_id parameter (the PM doing the merge) and use it as
the primary workspace owner. Falls back to task.assigned_to, then
created_by, before raising. cell_pm_complete now threads pm_agent_id
through.

roboco/services/gateway/choreographer.py

@@ -2145,7 +2145,9 @@ class Choreographer:
                 verb="cell_pm_complete",
             )
         target = parent_branch_for(t.branch_name)
-        merge_result = await self.git.pr_merge(t.pr_number, target=target)
+        merge_result = await self.git.pr_merge(
+            t.pr_number, target=target, actor_agent_id=pm_agent_id
+        )
         leaf_parent_id = t.parent_task_id
         leaf_team = t.team
         t = await self.task.cell_pm_complete(

roboco/services/git.py

@@ -1740,7 +1740,13 @@ class GitService(BaseService):
             .with_for_update(of=_TaskTable)
         )
 
-    async def pr_merge(self, pr_number: int, *, target: str) -> dict[str, Any]:
+    async def pr_merge(
+        self,
+        pr_number: int,
+        *,
+        target: str,
+        actor_agent_id: UUID | None = None,
+    ) -> dict[str, Any]:
         """Merge PR `pr_number` into `target`.
 
         Returns: ``{"merge_commit_sha": str | None}``. Looks up the
@@ -1768,9 +1774,18 @@ class GitService(BaseService):
         if project is None:
             raise NotFoundError("Project", str(task.project_id))
 
+        # Workspace resolution priority: caller-provided actor (the PM
+        # doing the merge) > task.assigned_to > created_by. assigned_to
+        # is often None at merge time because submit_qa / pass_qa cleared
+        # it during prior transitions; without a fallback the resolver
+        # raises ValidationError when project.workspace_path is unset.
+        workspace_agent_id = actor_agent_id or (
+            UUID(str(task.assigned_to)) if task.assigned_to else None
+        )
+        if workspace_agent_id is None and task.created_by:
+            workspace_agent_id = UUID(str(task.created_by))
         workspace = await self.get_workspace(
-            project.slug,
+            project.slug, agent_id=workspace_agent_id
-            agent_id=UUID(str(task.assigned_to)) if task.assigned_to else None,
         )
         git_token = await self._get_project_token_or_raise(project.slug)
         owner, repo = self._parse_github_remote(workspace)

tests/unit/gateway/test_choreographer_pm.py

@@ -263,7 +263,9 @@ async def test_cell_pm_complete_merges_then_completes() -> None:
     env = await c.cell_pm_complete(pm_id, task_id, notes="reviewed and approved")
     assert env.error is None
     assert env.status == "completed"
-    git_svc.pr_merge.assert_awaited_once_with(8, target="feature/backend/abc")
+    git_svc.pr_merge.assert_awaited_once_with(
+        8, target="feature/backend/abc", actor_agent_id=pm_id
+    )
 
 
 @pytest.mark.asyncio