feat: guarded merger lease adoption flow (#536)

Replace manual in-process _SESSION_LEASE seeding with an auditable
gitea_adopt_merger_pr_lease tool that posts durable adoption proof and
records sanctioned session provenance. Mutation gates now reject bare
record_session_lease calls without provenance from acquire/adopt/heartbeat.

Closes #536

tests/test_mcp_server.py

@@ -127,6 +127,7 @@ def _install_owned_reviewer_lease(
     session_id=_DEFAULT_LEASE_SESSION,
     head_sha="abc123",
 ):
+    import merger_lease_adoption as mla
     import reviewer_pr_lease
 
     reviewer_pr_lease.clear_session_lease()
@@ -135,7 +136,11 @@ def _install_owned_reviewer_lease(
         "session_id": session_id,
         "candidate_head": head_sha,
         "target_branch": "master",
-    })
+        "comment_id": 9001,
+    }, lease_provenance=mla.build_lease_provenance(
+        source=mla.SOURCE_ACQUIRE,
+        comment_id=9001,
+    ))
     return patch(
         "mcp_server._fetch_pr_comments",
         return_value=[