review(5): flag matrix-synapse stale-test status gap

machine-docs/BACKLOG-5.md

@@ -25,6 +25,32 @@ Single-writer: `## Build backlog` = Builder-only; `## Adversary findings` = Adve
 
 ## Adversary findings
 
+### [adversary] A5-4 — `matrix-synapse` stale-test/default path leaves no recipe commit status
+**Status:** OPEN — found 2026-06-01T14:16:00Z; see `REVIEW-5.md`.
+
+On the live V5 stale-test candidate `recipe-maintainers/matrix-synapse` PR `#1`, the PR comments show a
+terminal failed `!testme` result for build `#53` plus the default-mode explanatory stale-test comment,
+but the recipe PR head has **no** `cc-ci/testme` commit status at all. As a result, the helper cannot
+read the verdict back from the PR and poll-only returns `PENDING` even though the PR already shows the
+terminal outcome.
+
+**Cold repro:**
+1. Use `recipe-maintainers/matrix-synapse` PR `#1`, head
+   `21e5d84430bdc52f8fa8aa9a40fa5bda8adf06c0`.
+2. Confirm PR comments include:
+   - failure result comment for build `#53` (`#13872`), and
+   - explanatory stale-test comment (`#13877`).
+3. Run:
+   `POST=0 MAX_WAIT=20 INTERVAL=5 /srv/cc-ci/.claude/skills/recipe-upgrade/testme-on-pr.sh matrix-synapse 1`
+4. Observe:
+   - helper returns `VERDICT=PENDING` and `BUILD=?`;
+   - `GET /repos/recipe-maintainers/matrix-synapse/commits/21e5d84430bdc52f8fa8aa9a40fa5bda8adf06c0/status`
+     returns `{"state":"","total_count":0,"statuses":null}`.
+
+**Impact:** this breaks the Phase-5 requirement that the upgrade tooling read the verdict back from the
+PR on the live stale-test/default path. The comment surface says the run is terminal; the status surface
+still says nothing.
+
 ### [adversary] A5-3 — `POST=1 testme-on-pr.sh` can return a stale prior GREEN on re-runs
 **Status:** CLOSED — re-tested 2026-06-01T03:31:30Z; see `REVIEW-5.md` follow-up entry.