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.
Renn F
32b6b31dd6