fix(gateway): cell PM completes its own cell task; drop main-PM handoff

submit_up bubbled the cell task to Main PM (_handoff_to_main_pm), but
main_pm_complete rejects any task with a parent_task_id ("only operates
on root tasks"), so the cell->root PR had no one to merge it and the
cell task wedged at awaiting_pm_review. _maybe_advance_parent_to_pm_review
already intends the CELL PM to complete it.

Cell PM now owns cell completion:
- submit_up no longer hands off to Main PM; the cell task stays assigned
  to the cell PM, which is respawned to complete() it. Removed the
  now-unused _handoff_to_main_pm.
- cell_pm_complete resolves the merge target from the parent task's real
  branch_name (shared merge_chain.resolve_parent_branch, also used by the
  PR side-effects) so the cell->root PR merges into feature/main_pm/...,
  not the team-mis-derived feature/<cellteam>/... (same root cause as the
  prior PR-base fix).
- submit_up description + next_hint updated; lifecycle artifacts regen.

Main PM still only completes the ROOT (root->master + escalate-to-CEO).
First run to reach cell-PM bubble-up exposed this.

tests/unit/gateway/test_choreographer_pm.py

@@ -251,18 +251,25 @@ async def test_unblock_restore_false_returns_legacy_message() -> None:
 async def test_cell_pm_complete_merges_then_completes() -> None:
     pm_id = uuid4()
     task_id = uuid4()
+    parent_id = uuid4()
     t = MagicMock(
         id=task_id,
         status="awaiting_pm_review",
         assigned_to=pm_id,
         pr_number=8,
         branch_name="feature/backend/abc--def",
-        parent_task_id=uuid4(),
+        parent_task_id=parent_id,
         team="backend",
     )
     after = MagicMock(**{**t.__dict__, "status": "completed"})
+    # #181/#182: the merge target is the PARENT task's real branch_name —
+    # here under a DIFFERENT team prefix, which the old parent_branch_for
+    # would have mis-derived as feature/backend/abc.
+    parent = MagicMock(
+        id=parent_id, branch_name="feature/main_pm/abc", parent_task_id=None
+    )
     task_svc = AsyncMock()
-    task_svc.get.return_value = t
+    task_svc.get.side_effect = lambda tid: parent if tid == parent_id else t
     task_svc.all_subtasks_terminal.return_value = True
     task_svc.cell_pm_complete.return_value = after
     git_svc = AsyncMock()
@@ -278,7 +285,7 @@ async def test_cell_pm_complete_merges_then_completes() -> None:
     assert env.error is None
     assert env.status == "completed"
     git_svc.pr_merge.assert_awaited_once_with(
-        8, target="feature/backend/abc", actor_agent_id=pm_id
+        8, target="feature/main_pm/abc", actor_agent_id=pm_id
     )