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",
     )