feat: require proof-backed PR review handoff claims (Closes #395)

Reviewer final reports must not claim proof-sensitive workflow steps
unless each claim carries explicit command/tool evidence or structured
MCP result metadata:

- New reviewer_proof_backed_claims.py verifier covering the five claim
  families from the issue: inventory completeness (explicit pagination
  metadata required; page-size assumptions rejected), earlier-PR skips
  (per-PR command/tool or structured mergeability/blocker evidence),
  baseline validation (worktree path, target SHA, dirty-before, exact
  command, exact result, dirty-after), master integration (exact
  merge/rebase/simulation command), and cleanup (final git worktree
  list proof).
- PROOF_PROVENANCE_CLASSES constant distinguishes command actually run,
  MCP metadata, inherited from previous blocker state, and not checked.
- Re-export from review_proofs and wire into build_final_report as a
  downgrade check with proof_backed_claims_proven/violations fields.
- New workflow section 30A documents the rule; contract test locks the
  marker and an export test locks the re-export.
- 13 tests in tests/test_reviewer_proof_backed_claims.py covering the
  required scenarios: baseline claim without command, skipped-PR claim
  without evidence, inventory-complete from page-size assumption only,
  cleanup without final worktree proof, and a fully proof-backed report
  passing.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

tests/test_llm_workflow_split.py

@@ -61,6 +61,7 @@ def test_review_merge_workflow_contract():
     assert "## 0. Load the canonical workflow first" in text
     assert "## 26A. Terminal review mutation hard-stop" in text
     assert "## 11A. Skipped PRs are read-only" in text
+    assert "## 30A. Proof-backed claims rule (#395)" in text
     assert "## 35. Duplicate request-changes prevention" in text
     assert "## 37. Controller handoff schema" in text
     assert "INVENTORY_PAGINATION_UNPROVEN" in text
@@ -219,6 +220,12 @@ def test_inventory_worktree_verifier_exported():
     assert callable(assess_inventory_worktree_report)
 
 
+def test_proof_backed_claims_verifier_exported():
+    from review_proofs import assess_proof_backed_handoff_report
+
+    assert callable(assess_proof_backed_handoff_report)
+
+
 def test_infra_stop_handoff_verifier_exported():
     from review_proofs import assess_infra_stop_handoff_report