feat: add issue claim heartbeat leases and stale-claim reconciliation (Closes #268)

Structured claim/progress heartbeats post on issue claim and via
gitea_post_heartbeat. Read-only gitea_reconcile_issue_claims inventories
active, stale, phantom, and PR-backed claims; gitea_cleanup_stale_claims
supports dry-run cleanup of reclaimable labels.

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

tests/test_mcp_server.py

@@ -321,15 +321,17 @@ class TestMarkIssue(unittest.TestCase):
     @patch("mcp_server.api_request")
     @patch("mcp_server.get_auth_header", return_value=FAKE_AUTH)
     def test_start_adds_label(self, _auth, mock_api):
-        # First call: get labels; second call: add label
+        # labels, add label, post claim heartbeat comment
         mock_api.side_effect = [
             [{"id": 10, "name": "status:in-progress"}],
             [{"name": "status:in-progress"}],
+            {"id": 99},
         ]
         with patch.dict(os.environ, ISSUE_WRITE_ENV, clear=True):
             result = gitea_mark_issue(issue_number=5, action="start")
         self.assertTrue(result["success"])
         self.assertIn("claimed", result["message"])
+        self.assertTrue(result.get("heartbeat_posted"))
 
     @patch("mcp_server.api_request")
     @patch("mcp_server.get_auth_header", return_value=FAKE_AUTH)