The root loop-stopper (F016) was root-only; a weak cell PM could re-submit
the unchanged cell->root PR after a pr_fail and loop awaiting_pr_review ->
pr_fail forever (the cell analogue of the 2026-06-27 root loop).
pr_fail stamps the assembled PR's head SHA into notes_structured.pr_review
.head_sha for cell AND root gate tasks alike (the capture is gate-verb-
level, not root-level), so the same structural refusal applies to submit_up:
if the cell PR's current head SHA equals the SHA the last pr_fail recorded,
no new dev work landed on the cell branch -> the diff is byte-identical ->
refuse, do not re-open the gate. Different SHA -> branch advanced -> allow.
- _submit_up_unchanged_pr_guard mirrors _submit_root_unchanged_pr_guard
(cell-PM remediation: re-delegate to the dev + wait for re-assembly),
wired into submit_up after _submit_up_guard passes.
- Renamed shared _current_root_pr_head_sha -> _current_pr_head_sha (both
guards use it; the lookup was never root-specific).
- Every ambiguous case FAILS OPEN (no prior fail, no recorded sha, no
pr_number, no resolvable project, git/closed-PR None) — only the exact-
unchanged case is hard-blocked.
TDD red->green; ruff + mypy clean; F007+F016 guard suites green (15 passed).
