feat: require validation cwd and HEAD proof in reviewer reports (Closes #398)

Add assess_validation_cwd_proof_report so reviewer final reports must prove
pwd, git rev-parse HEAD, git status, and expected PR head SHA before
claiming validation ran in a branches/ worktree. Wire the verifier into
final_report_validator and gitea_validate_review_final_report.

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

review_final_report_schema.py

@@ -301,6 +301,7 @@ def assess_review_final_report_schema(
     action_log: list[dict] | None = None,
     mutations_observed: bool = False,
     local_edits: bool = False,
+    validation_session: dict | None = None,
 ) -> dict[str, Any]:
     """Validate reviewer final report text before session completion (#391)."""
     base = assess_final_report_validator(
@@ -312,6 +313,7 @@ def assess_review_final_report_schema(
         action_log=action_log,
         mutations_observed=mutations_observed,
         local_edits=local_edits,
+        validation_session=validation_session,
     )
     extra: list[dict[str, str]] = []
     for rule in _SCHEMA_RULES: