feat(review-workflow): add fail-closed proofs for blind PR queue review (#173)

Adds review_proofs.py with pure, fail-closed helpers for the reviewer-side
blind queue workflow:

- verify_pinned_head_checkout: proves HEAD == pinned PR head (full 40-hex,
  no prefix matches) and diff base == PR base; any mismatch blocks
  review/merge.
- assess_inventory_completeness: exhaustive "only PRs found" claims require
  every configured repo listed with stated filters, proven pagination, and
  reported open-PR totals.
- assess_validation_report: validation results are unclaimable unless the
  exact command was stated and its output read; missing pass/fail/skip
  counts, unjustified ignored paths, or unexplained deviation from the
  canonical command downgrade the report to weak.
- assess_self_review_contamination: contamination must be evidence-backed;
  bare same-session claims (either way) yield "unknown" with a
  choose-another-PR-or-stop action, never an assumed verdict.
- build_final_report: distinguishes identity eligibility, author/reviewer
  distinctness, session contamination, validation-on-pinned-head, merge,
  and issue verification; grades "A" only when every proof is present,
  downgrades otherwise, and marks a merge without allowing proofs as a
  blocked violation.

tests/test_review_proofs.py covers all seven harness assertions from the
issue (unchecked-out pinned head blocks merge; multi-repo inventory;
pagination; missing counts flagged weak; evidence-required contamination;
unread output cannot claim validation passed; ignored paths need
justification).

SKILL.md section F and templates/review-pr.md now require the checkout,
inventory, validation-evidence, and contamination proofs and the
distinguished final report. No review/merge safety gate is weakened.

Closes #173

Co-Authored-By: Claude Fable 5 <[email protected]>

skills/llm-project-workflow/SKILL.md

@@ -167,11 +167,38 @@ Worktree folder = branch with `/` replaced by `-`
 
 1. Use a separate review worktree (`scripts/worktree-review <branch>`), detached.
 2. Verify your authenticated identity.
-3. Verify the PR author — **you must not be the author.**
+3. Verify the PR author — **you must not be the author.** Self-review
+   contamination must be *evidence-backed* (#173): state the authenticated
+   reviewer identity, the PR author identity, whether this session
+   authored/touched the PR branch, and the evidence source for any
+   "same-session author" claim. If the evidence is missing, report the
+   status as **unknown** — never declare contamination by assumption — and
+   choose another PR or stop (`review_proofs.assess_self_review_contamination`).
 4. Verify the worktree is clean.
-5. Inspect the full diff; confirm scope matches the linked issue; flag unrelated files.
-6. Run the tests.
-7. **Do not merge if checks fail. Do not merge if the reviewer is the author.**
+5. **Checkout proof (#173):** before reviewing or validating, prove and
+   state: the selected PR head SHA from Gitea (pinned), the local checkout
+   SHA (`git rev-parse HEAD`), that `HEAD ==` the pinned PR head SHA, and
+   that the diff base is the PR base branch. If `HEAD` does not match the
+   pinned head, **stop before review/merge**
+   (`review_proofs.verify_pinned_head_checkout`).
+6. **Inventory proof (#173):** a blind queue review must prove listing
+   completeness before claiming "only PRs found": both configured
+   repositories checked, open-PR filters stated, pagination handled or
+   explicitly not needed, and the total open PR count per repo reported
+   (`review_proofs.assess_inventory_completeness`).
+7. Inspect the full diff; confirm scope matches the linked issue; flag unrelated files.
+8. Run the tests. Validation reporting must include the exact command and
+   exact results: pass/fail, counts of tests passed/skipped/failed, any
+   ignored paths and why they are safe to ignore, and whether the command
+   differs from the repository's canonical validation command. Only claim a
+   validation result after the command has completed and its output has
+   been read (`review_proofs.assess_validation_report`).
+9. **Do not merge if checks fail. Do not merge if the reviewer is the author.**
+10. The final report must distinguish (`review_proofs.build_final_report`):
+    identity eligible; PR author different from reviewer; session
+    contamination absent (with evidence); validation performed on the pinned
+    head; merge performed; issue status verified. If any proof is missing,
+    stop or downgrade the result instead of merging confidently.
 
 ## G. Merge / cleanup workflow