feat(gateway): complete openclaw phase1 queue parity v2

docs/api/PROTOCOL.md

@@ -33,6 +33,8 @@ The gateway serialises agent work **per session**, not per WebSocket connection:
 
 - Requests that target the same `sessionId` run one-at-a-time (FIFO) in a per-session lane.
 - Requests for different sessions can run in parallel.
+- Lane policy is configurable (`collect`, `followup`, `steer_backlog`, `interrupt`) with per-channel and per-session overrides.
+- Session-local overrides can be managed at runtime via `agent.send` commands: `/queue`, `/queue set ...`, `/queue reset`.
 
 This is implemented via a per-lane queue (`LaneQueue`) in the gateway server, and used by `agent.send` and `agent.cancel`.
 
@@ -555,6 +557,8 @@ Send a message to the agent and stream response.
 }
 ```
 
+When queue policy rejects/supersedes a request before execution, the server emits an `error` event with `code: 3` (`AgentBusy`) and includes `data.queue` metadata (`code`, `laneId`, `mode`, `overflow`, `droppedCount`).
+
 #### `agent.cancel`
 
 Cancel the current agent operation.
@@ -876,11 +880,20 @@ Error occurred during processing.
   "event": "error",
   "data": {
     "code": 5,
-    "message": "Internal error: ..."
+    "message": "Internal error: ...",
+    "queue": {
+      "code": "overflow",
+      "laneId": "ws:abc123",
+      "mode": "followup",
+      "overflow": "drop_new",
+      "droppedCount": 1
+    }
   }
 }
 ```
 
+`data.queue` is optional and only present for queue policy rejections/superseded requests.
+
 ## Error Codes
 
 | Code | Name | Description |