fix(agentctl): re-lock write txn after schema DDL for claim CAS

scripts/agentctl.py

@@ -588,13 +588,18 @@ class Repo:
             db.execute('PRAGMA busy_timeout=5000')
             if write:
                 db.execute('PRAGMA synchronous=FULL')
+                # DDL inside ensure_schema implicitly commits any open txn; take the
+                # exclusive lock only after schema is stable so claim/lease CAS holds.
                 db.execute('BEGIN IMMEDIATE')
                 ensure_schema(db)
+                if db.in_transaction:
+                    db.execute('COMMIT')
+                db.execute('BEGIN IMMEDIATE')
             yield db
-            if write:
+            if write and db.in_transaction:
                 db.commit()
         except BaseException:
-            if write:
+            if write and db.in_transaction:
                 db.rollback()
             raise
         finally: