fix(mcp): let a sanctioned recovery keep its own owning PR (Closes #755)

#753 added the dead-session author-lock recovery assessor, but no sanctioned
recovery could ever reach the lock write. A dead-session lock is by construction
a lock for work that already has an open PR, and the #400 duplicate-work gate
blocked unconditionally on any linked open PR. gitea_lock_issue computed
recovery_sanctioned, then discarded it one gate later:

    open PR #750 already covers issue #749 (fail closed)

Because the recovered lock was never persisted, it stayed non-live, so
_prove_author_ownership_for_pr found no live author lock and
gitea_update_pr_branch_by_merge failed closed too. Both blockers had a single
cause, so only the first one is fixed here.

issue_lock_recovery.owning_pr_recovery_evidence distils a granted assessment
into the minimum evidence the duplicate gate needs: issue, branch, owning PR
number, and head. It returns None unless the outcome is RECOVERY_SANCTIONED and
the assessment's own evidence agrees that local head == remote head == PR head,
so a partial or hand-built assessment cannot authorize anything.

The duplicate gate takes that evidence and re-checks every element against the
live PR list it was handed: exactly one linked open PR, matching number, head
branch, head SHA, and locked branch. Only that exact self-owned PR is exempt.
A different PR, several linked PRs, a different branch or head, or evidence for
another issue all keep failing closed, with a diagnostic naming which element
disagreed. The competing-branch, claim-lease and commit/push/create_pr arms are
untouched, and with no evidence the gate behaves exactly as before.

Ownership proof needed no change: once the recovered lock persists under the
live session pid, _prove_author_ownership_for_pr matches it as before.

Out of scope: the PHASE_COMMIT/PHASE_PUSH/PHASE_CREATE_PR rechecks still block
on a linked open PR for every author, recovered or not. That is pre-existing
#400 behavior, unrelated to lock recovery, and #755 does not cover it.

Tests
- tests/test_issue_755_owning_pr_recovery.py (new, 31 tests) drives the real
  mcp_server.gitea_lock_issue handler, not only the pure assessor: sanctioned
  recovery relocks, persists a live lease with truthful dead_session_recovery
  provenance, and satisfies _prove_author_ownership_for_pr; competing PR,
  multiple linked PRs, mismatched head/branch/worktree, dirty worktree, live
  prior pid, and a fresh claim with no prior lock all stay blocked.
- Neutralizing owning_pr_recovery_evidence regresses all three success tests to
  the exact production error above, so the coverage is load-bearing.
- Focused suites (755, 753, duplicate gates, lock registration, provenance) —
  102 passed.
- Lock/ownership/worktree/handoff suites — 172 passed, 16 subtests.
- Full suite tests/ — 3529 passed, 6 skipped, 2 failed, 365 subtests.
- The same 2 failures reproduce on a pristine detached worktree at
  08f67007c5 (3498 passed, 6 skipped, 2 failed):
  test_issue_702_review_findings_f1_f6 F1 recovery-before-probe and
  test_reconciler_supersession_close org/repo forwarding. Pre-existing.
- git diff --check clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_017BNsk3KUuFchaZyPJsCxjk

issue_lock_recovery.py

@@ -362,6 +362,57 @@ def _result(
     }
 
 
+def owning_pr_recovery_evidence(
+    assessment: Mapping[str, Any] | None,
+) -> dict[str, Any] | None:
+    """Server-derived proof of the open PR a sanctioned recovery already owns (#755).
+
+    A dead-session recovery is, by construction, recovery of work that already
+    has an open PR — so the duplicate-work gate's linked-open-PR blocker would
+    otherwise discard every sanctioned recovery. This distils the completed
+    assessment into the minimum evidence that gate needs to tell "the PR this
+    lock already owns" apart from "a competing duplicate PR".
+
+    Returns ``None`` unless recovery was actually granted and the assessment's
+    own evidence names exactly one owning PR whose head agrees with the local
+    and remote heads. Nothing here is caller-supplied: every field is copied
+    from evidence the assessor built out of durable lock state plus live
+    git/Gitea observation, so a caller cannot manufacture an exemption.
+    """
+    if not isinstance(assessment, Mapping):
+        return None
+    if assessment.get("outcome") != RECOVERY_SANCTIONED:
+        return None
+    if not assessment.get("recovery_sanctioned"):
+        return None
+
+    evidence = assessment.get("evidence") or {}
+    branch_name = _text(evidence.get("locked_branch"))
+    pr_head = _text(evidence.get("pr_head"))
+    local_head = _text(evidence.get("local_head"))
+    remote_head = _text(evidence.get("remote_head"))
+    raw_pr_number = evidence.get("pr_number")
+
+    if raw_pr_number is None or not branch_name or not pr_head:
+        return None
+    # The assessor already required these to agree. Re-check, so a truncated or
+    # hand-built evidence map can never authorize an exemption.
+    if pr_head != local_head or pr_head != remote_head:
+        return None
+    try:
+        pr_number = int(raw_pr_number)
+        issue_number = int(evidence.get("issue_number"))
+    except (TypeError, ValueError):
+        return None
+
+    return {
+        "issue_number": issue_number,
+        "pr_number": pr_number,
+        "branch_name": branch_name,
+        "head_sha": pr_head,
+    }
+
+
 def build_recovery_record(
     assessment: Mapping[str, Any],
     *,