fix(gateway): PR base/target is the parent task's branch, not derived

submit_up opened the cell->root PR with a base computed by
merge_chain.parent_branch_for, which drops the last --segment but
REUSES the child's team prefix. Across a team boundary (cell
feature/backend/ROOT--CELL -> root feature/main_pm/ROOT) that yields a
ref that does not exist on the remote, so GitHub rejects the PR with
422 base: invalid and the cell parent wedges.

Resolve the base/target from the parent task's authoritative
branch_name (what branch creation already cuts each child from), via a
shared VerbRunner._parent_branch_for helper used by _do_create_pr and
_do_pr_merge. Falls back to parent_branch_for only when there is no
parent (root->master -> master) or it has no branch yet. The leaf->cell
path is unchanged (same team). Latent since the merge chain landed;
first run to reach cell-PM bubble-up exposed it after #180.

tests/unit/gateway/test_choreographer_pm_extras.py

@@ -890,6 +890,7 @@ async def test_submit_up_opens_pr_and_reassigns_to_main_pm() -> None:
         id=task_id,
         status="in_progress",
         assigned_to=pm_id,
+        parent_task_id=None,
         branch_name="feature/backend/abc123",
         team="backend",
     )
@@ -897,6 +898,7 @@ async def test_submit_up_opens_pr_and_reassigns_to_main_pm() -> None:
         id=task_id,
         status="awaiting_pm_review",
         assigned_to=pm_id,
+        parent_task_id=None,
         branch_name="feature/backend/abc123",
         team="backend",
     )

tests/unit/gateway/test_open_pr.py

@@ -70,6 +70,10 @@ async def test_open_pr_pushes_and_opens_pr() -> None:
         plan="x",
         commits=[{"sha": "abc"}],
         pr_number=None,
+        # No parent → _do_create_pr falls back to parent_branch_for; this
+        # test asserts push+create mechanics, not the cell→root base (#181,
+        # covered in test_verb_runner).
+        parent_task_id=None,
         branch_name="feature/backend/abc12345",
     )
     # Re-fetched task post-runner has the new pr_number written by
@@ -82,6 +86,7 @@ async def test_open_pr_pushes_and_opens_pr() -> None:
         commits=[{"sha": "abc"}],
         pr_number=42,
         pr_url="https://gh/x/42",
+        parent_task_id=None,
         branch_name="feature/backend/abc12345",
     )
     task_svc = AsyncMock()

tests/unit/gateway/test_verb_runner.py

@@ -70,6 +70,7 @@ async def test_runner_runs_side_effects_after_db_commit() -> None:
         status="in_progress",
         commits=["abc"],
         pr_number=None,
+        parent_task_id=None,
         branch_name="feature/backend/ABC12345",
     )
     agent = MagicMock(id=uuid4(), role="developer")
@@ -115,6 +116,7 @@ async def test_submit_up_creates_pr_before_transition() -> None:
     task = MagicMock(
         id=uuid4(),
         status="in_progress",
+        parent_task_id=None,
         branch_name="feature/backend/ABC12345--DEF67890",
     )
     agent = MagicMock(id=uuid4(), role="cell_pm")
@@ -126,6 +128,53 @@ async def test_submit_up_creates_pr_before_transition() -> None:
     )
 
 
+@pytest.mark.asyncio
+async def test_create_pr_base_is_parent_task_branch_across_team() -> None:
+    """#181: the cell→root PR base is the PARENT task's actual branch_name,
+    not the team-preserving string derivation.
+
+    The cell branch is feature/backend/ROOT--CELL but the root branch is
+    feature/main_pm/ROOT (different team). parent_branch_for() would derive
+    feature/backend/ROOT — a ref that doesn't exist on the remote, which
+    GitHub rejects with base: invalid. The base must come from the parent
+    task's branch_name.
+    """
+    task_svc = AsyncMock()
+    task_svc.session.begin_nested = MagicMock(
+        return_value=MagicMock(__aenter__=AsyncMock(), __aexit__=AsyncMock())
+    )
+    task_svc.submit_pm_review = AsyncMock(
+        return_value=MagicMock(status="awaiting_pm_review")
+    )
+    root_id = uuid4()
+    # Parent (root) task lives under a DIFFERENT team prefix.
+    task_svc.get = AsyncMock(
+        return_value=MagicMock(branch_name="feature/main_pm/ROOT0001")
+    )
+
+    git_svc = AsyncMock()
+    git_svc.create_pr = AsyncMock(return_value={"pr_number": 32})
+    runner = VerbRunner(task_service=task_svc, git_service=git_svc)
+
+    task = MagicMock(
+        id=uuid4(),
+        status="in_progress",
+        parent_task_id=root_id,
+        branch_name="feature/backend/ROOT0001--CELL0001",
+    )
+    agent = MagicMock(id=uuid4(), role="cell_pm")
+    ctx = spec.Context(notes="cell scope complete; bubbling up to main pm")
+
+    await runner.run_intent("submit_up", task, agent, ctx)
+
+    task_svc.get.assert_awaited_once_with(root_id)
+    _, kwargs = git_svc.create_pr.call_args
+    assert kwargs["parent"] == "feature/main_pm/ROOT0001", (
+        "cell→root PR base must be the parent task's real branch, not the "
+        f"team-derived name; got parent={kwargs['parent']!r}"
+    )
+
+
 def test_submit_up_spec_pins_pr_before_transition() -> None:
     """The submit_up spec declares create_pr as a pre_side_effect, not a
     trailing side_effect — the ordering fix lives in the spec."""