A rewound turn's id is forgotten, so sending it again runs the turn anew

The turn log answered a repeated id with what that turn had answered, which
was right for a prompt sent twice over a dropped link and wrong for a turn
the web UI had just rewound to edit: the same id came back with the old
reply. rewind now returns the ids it took out, both handlers forget them from
the log, and the turn file drops them too, or the next load would have
brought them back.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/acp/agent.ts

@@ -1008,7 +1008,8 @@ export class AcpAgent {
     if (s.prompting || s.turns.queued().length) throw new RpcError(INVALID_PARAMS, "this session is working — rewind it when the reply is done")
     const reverted = s.app.turns.rewind(turnId)
     if (reverted === undefined) throw new RpcError(INVALID_PARAMS, `this session has no turn ${turnId}`)
-    return { reverted }
+    s.turns.forget(reverted)
+    return { reverted: reverted.length }
   }
 
   /** `_lembas/session/delete`: the chat was deleted in the web UI, so the session is too —