feat: comment-then-stop policy for partial PR reconciliation (Closes #302)

Already-landed PR reconciliation could prove a PR should be closed but
stop silently when gitea.pr.close was unavailable, even with comment
capability present. Adopt Option B (comment-then-stop) as the durable
policy and enforce it:

- resolve_partial_reconciliation_plan maps proven capabilities to one
  of four outcomes: FULL_RECONCILE_CLOSE_ALLOWED (close_pr proven),
  PARTIAL_RECONCILE_COMMENT_THEN_STOP (comment_pr proven, close_pr
  missing; one reconciliation comment with PR head SHA, target branch
  SHA, ancestor proof, linked issue status, and the missing capability,
  then stop), RECOVERY_HANDOFF_ONLY (no comment capability; no Gitea
  mutation), and GATE_NOT_PROVEN (no ancestry proof; no mutation
  regardless of capability). Missing capability dicts fail closed.
- assess_partial_reconciliation_report requires final reports to
  explain why the comment was or was not posted, name the missing
  capability, and state the close result or that no mutation ran.
- workflows/reconcile-landed-pr.md gains section 12A documenting the
  policy; the workflow contract test locks the new markers.

Tests cover close capability present, close missing with comment
allowed, comment capability missing, all capabilities missing,
unproven ancestry, fail-closed capability dict, and report checks for
each outcome.

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

tests/test_llm_workflow_split.py

@@ -78,6 +78,12 @@ def test_reconcile_landed_workflow_contract():
     assert "canonical: true" in text
     assert "Do not review or merge normal PRs" in text
     assert "Already-landed proof" in text
+    # Issue #302: partial reconciliation policy must stay documented.
+    assert "## 12A. Partial reconciliation policy (#302)" in text
+    assert "comment-then-stop" in text
+    assert "PARTIAL_RECONCILE_COMMENT_THEN_STOP" in text
+    assert "RECOVERY_HANDOFF_ONLY" in text
+    assert "resolve_partial_reconciliation_plan" in text
 
 
 def test_create_issue_workflow_contract():