feat(gateway): support per-channel and per-session queue policy overrides

README.md

@@ -807,6 +807,15 @@ server:
     mode: collect      # collect | steer | interrupt
     cap: 50            # max pending requests per session lane
     overflow: drop_old # drop_old | drop_new
+    overrides:
+      channels:
+        ws:
+          mode: steer
+          cap: 10
+      sessions:
+        ws:vip-user:
+          mode: interrupt
+          overflow: drop_new
 ```
 
 Notes:
@@ -814,6 +823,7 @@ Notes:
 - `steer` and `interrupt` keep only the latest pending request while one is active.
 - `interrupt` currently does not force-stop already running work; use `agent.cancel` for active cancellation.
 - On overflow, `drop_old` evicts the oldest pending request, `drop_new` rejects the new request.
+- Override precedence: exact `sessions` match first, then `channels`.
 
 ## Gateway Request Body Limit