feat: add own-branch lock adoption recovery to gitea_lock_issue (Closes #442)

Closes #442.

gitea_lock_issue treated ANY remote branch containing issue-<n> as
duplicate competing work (#400) and failed closed — including the issue's
own already-pushed branch. If the in-memory lock was lost (e.g. MCP server
restart) after the branch was pushed but before the PR, there was no
non-destructive way to reacquire the lock, deadlocking PR creation. This
surfaced during recovery of #420 (feat/issue-420-server-code-parity).

Adds a safe own-branch adoption path.

Changes:
- issue_lock_adoption.py — assess_own_branch_adoption() distinguishes the
  issue's exact requested branch (adopt) from a different same-issue branch
  (competing, still fail-closed); ambiguous (own + other) fails closed.
  build_adoption_proof() emits the required proof block.
- gitea_mcp_server.py — gitea_lock_issue consults the adoption assessment in
  place of the blanket branch-match block; on adoption the result carries an
  `adoption` proof (issue, branch, head commit, reason, no-existing-PR proof,
  no-competing-live-lock proof, lock file path/status). Open-PR and active-
  lease checks already run first, so adoption implies neither exists. Adds
  _branch_entry_commit_sha helper.
- Tests: tests/test_issue_lock_adoption.py (8 cases) + 3 MCP-level cases in
  test_mcp_server.py (adopt own branch, open PR blocks adoption, create_pr
  proceeds after adopted lock). Existing competing-branch block preserved
  (still raises "already has matching branch").

Safety:
- Different same-issue branch, competing branch, existing open PR, ambiguous
  ownership all remain fail-closed. #400 duplicate-work protection intact.
- #420's feature branch was only read (git ls-remote); not modified/deleted.

Validation:
  venv/bin/python -m pytest tests/ -q  ->  1611 passed, 6 skipped

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

gitea_mcp_server.py

@@ -503,6 +503,7 @@ import issue_lock_worktree  # noqa: E402
 import already_landed_reconcile  # noqa: E402
 import author_mutation_worktree  # noqa: E402
 import issue_claim_heartbeat  # noqa: E402
+import issue_lock_adoption  # noqa: E402
 import merged_cleanup_reconcile  # noqa: E402
 import reconciler_profile  # noqa: E402
 import reconciliation_workflow  # noqa: E402
@@ -641,6 +642,19 @@ def _branch_entry_name(branch: dict | str) -> str:
     return str(branch.get("name") or branch.get("ref") or "")
 
 
+def _branch_entry_commit_sha(branch: dict | str) -> str | None:
+    """Best-effort head SHA for a Gitea branch entry (None when absent)."""
+    if not isinstance(branch, dict):
+        return None
+    commit = branch.get("commit")
+    if isinstance(commit, dict):
+        sha = commit.get("id") or commit.get("sha")
+        if sha:
+            return str(sha)
+    sha = branch.get("commit_sha")
+    return str(sha) if sha else None
+
+
 def _reveal_endpoints() -> bool:
     """Admin/debug opt-in (#120): include endpoint URLs and token source
     names in tool output. Off by default so normal LLM-facing responses
@@ -1146,13 +1160,28 @@ def gitea_lock_issue(
         branches = api_get_all(branch_url, auth)
     except Exception as e:
         raise RuntimeError(f"Could not list branches to verify issue lock: {e}")
-    for branch in branches:
-        name = _branch_entry_name(branch)
-        if expected_pattern in name:
-            raise ValueError(
-                f"Issue #{issue_number} already has matching branch '{name}' "
-                "(fail closed)"
-            )
+    # #442: distinguish the issue's own already-pushed branch (adoptable for
+    # lock recovery) from a different same-issue branch (competing work, still
+    # fail-closed per #400). Open-PR and active-lease checks already ran above,
+    # so reaching here means no open PR and no competing live lock.
+    existing_branch_entries = [
+        {
+            "name": _branch_entry_name(branch),
+            "commit_sha": _branch_entry_commit_sha(branch),
+        }
+        for branch in branches
+    ]
+    adoption = issue_lock_adoption.assess_own_branch_adoption(
+        issue_number=issue_number,
+        requested_branch=branch_name,
+        existing_branches=existing_branch_entries,
+    )
+    if adoption["block"]:
+        competing = ", ".join(adoption["competing_branches"])
+        raise ValueError(
+            f"Issue #{issue_number} already has matching branch '{competing}' "
+            "that is not the requested branch (fail closed)"
+        )
 
     work_lease = _build_author_issue_work_lease(
         issue_number=issue_number,
@@ -1190,6 +1219,21 @@ def gitea_lock_issue(
         "worktree_path": resolved_worktree,
         "work_lease": work_lease,
     }
+    if adoption["adopt"]:
+        # #442: recovery lock over the issue's own already-pushed branch.
+        result["adoption"] = issue_lock_adoption.build_adoption_proof(
+            issue_number=issue_number,
+            branch_name=branch_name,
+            assessment=adoption,
+            open_pr_checked=True,
+            competing_lock_checked=True,
+            lock_file_path=ISSUE_LOCK_FILE,
+            lock_file_status="written",
+        )
+        result["message"] = (
+            f"Adopted existing branch '{branch_name}' and locked issue "
+            f"#{issue_number} for recovery (fail-closed check complete)."
+        )
     if agent_artifacts:
         result["warnings"] = [
             "Agent temp artifacts at repo root (delete before implementation): "