feat(vela): retire mocked.turn.trigger from shared contract

docs/protocol.md

@@ -41,7 +41,6 @@ This increment intentionally keeps the envelope minimal:
 ```ts
 type ClientEvent =
   | { type: "session.start"; payload: {} }
-  | { type: "mocked.turn.trigger"; payload: {} }
   | { type: "input_audio.append"; payload: { chunk: string } }
   | { type: "input_audio.commit"; payload: {} }
   | { type: "response.cancel"; payload: {} };
@@ -50,11 +49,15 @@ type ClientEvent =
 #### Client event intent
 
 - `session.start` initializes a voice session without locking in transport or auth details yet
-- `mocked.turn.trigger` is a retired legacy event name that the gateway now rejects with a deterministic recoverable error
 - `input_audio.append` carries a chunk of captured input audio as an encoded string
 - `input_audio.commit` marks the current buffered user turn as ready for downstream processing
 - `response.cancel` interrupts the active listen/think/speak flow
 
+Legacy compatibility note:
+
+- `mocked.turn.trigger` is no longer part of the active shared client contract
+- the gateway still handles raw incoming `mocked.turn.trigger` envelopes explicitly and rejects them with a deterministic recoverable error for backward compatibility
+
 ### Current skeleton behavior
 
 - on connect, the gateway creates an ephemeral in-memory session and emits `session.ready` plus `session.state`