fix(gastown): use bare column names in Rule 1b subquery

The aliased table 'am' combined with the qualified interpolator
agent_metadata.bead_id produced 'am.agent_metadata.bead_id' (double-
qualified). Use agent_metadata.columns.bead_id for the bare column
name with the manual 'am.' prefix.

Author: jrf0110

cloudflare-gastown/src/dos/town/reconciler.ts

@@ -635,8 +635,8 @@ export function reconcileBeads(sql: SqlStorage, opts?: { draining?: boolean }):
           AND b.${beads.columns.rig_id} IS NOT NULL
           AND NOT EXISTS (
             SELECT 1 FROM ${agent_metadata} am
-            WHERE am.${agent_metadata.bead_id} = b.${beads.columns.assignee_agent_bead_id}
-              AND am.${agent_metadata.current_hook_bead_id} = b.${beads.columns.bead_id}
+            WHERE am.${agent_metadata.columns.bead_id} = b.${beads.columns.assignee_agent_bead_id}
+              AND am.${agent_metadata.columns.current_hook_bead_id} = b.${beads.columns.bead_id}
           )
       `,
       []