[fix] submit_root: hard unchanged-PR gate stops the pr_fail re-submit loop

The 2026-06-27 infinite pr_fail loop: a Main-PM root (PR #139) was pr_fail'd,
routed to needs_revision, and re-submitted byte-identical → awaiting_pr_review
→ pr_fail again, forever. The prior hint/a2a steer was ignored by the weak
coordinator model — hints don't stop a model that won't read them. A HARD gate
refuses the re-submit when the assembled root PR's head SHA is unchanged since
the last pr_fail (no new cell work → identical diff); a different SHA ⇒ the
branch advanced ⇒ allow. Every ambiguous case fails open (no prior fail, no
recorded SHA, no pr_number, unresolvable slug, git error, closed PR) — only the
exact-unchanged case is hard-blocked.

- content/models: PrReviewContent.head_sha (optional; JSON col → no migration).
- git: get_pr_head_sha (GitHub pulls API; None on any failure → fail-open).
- pr_gate: pr_fail captures head_sha into the verdict record; pr_pass does not.
- _impl: submit_root runs _submit_root_unchanged_pr_guard after _submit_up_guard;
  _current_root_pr_head_sha resolves slug + current SHA (fail-open).
- pr_review: extract module-level resolve_task_project_slug, shared by the mixin
  and the gate helper (_LegacyChoreographer reaches it via cast to the
  ChoreographerHelpers typed view — it doesn't inherit the helpers mixin).
- tests: test_submit_root_unchanged_pr_guard (11 — refuse/allow/6 fail-open/3
  capture-side, mypy-clean via cc:Any spy idiom, zero type:ignore) +
  test_pr_gate_notifies_pm capture-path stub.

tests/unit/gateway/test_pr_gate_notifies_pm.py

@@ -63,6 +63,14 @@ def _stub_gate_path(
         )
     )
     c._gate_tracing = AsyncMock(return_value=None)  # type: ignore[method-assign]
+    # These tests exercise the pr_fail a2a / notify path, not the head-sha
+    # capture (which has its own suite in test_submit_root_unchanged_pr_guard).
+    # Stub the capture so it does not walk the mock session into un-awaited
+    # coroutines; the verdict still lands via the _record_gate_verdict spy.
+    # Alias to ``Any`` so this addition needs no type:ignore (mypy doesn't flag
+    # attribute assignment on ``Any``; avoids ruff B010's no-setattr rule too).
+    cc: Any = c
+    cc._capture_pr_head_sha = AsyncMock(return_value=None)
     c._record_gate_verdict = MagicMock()  # type: ignore[method-assign]
     c._post_gate_review_to_pr = AsyncMock()  # type: ignore[method-assign]
     runner = MagicMock()