Fix agent runner host migration and runtime guards

Migrates legacy runner blocks into plugin runner configs, preserves run-scoped history boundaries, enforces operation/file authorization, and sanitizes inline attachment persistence. Also fixes plugin runner form dirty handling and adds regression coverage.

src/langbot/pkg/agent/runner/persistent_state_store.py

@@ -361,10 +361,8 @@ class PersistentStateStore:
                 delete(AgentRunnerState)
                 .where(AgentRunnerState.scope_key == scope_key)
                 .where(AgentRunnerState.state_key == state_key)
-                .returning(AgentRunnerState.id)
             )
-            deleted = result.first()
-            return deleted is not None
+            return (result.rowcount or 0) > 0
 
     async def state_list(
         self,