feat(slice-3): +rutster-brain-realtime crate skeleton + async-trait dep (spec §1.1)

Workspace member 8 of 8. Library + binary; mirrored slice-2's
rutster-tap-echo shape. Three stub modules (api_key, translator,
openai_client) filled in by Tasks 3-5. No behavioral code yet —
this task only locks the crate boundary + the new async-trait dep
(the Tool trait in Task 6 needs it for async-fns-in-trait-objects).

Cargo.lock

@@ -1182,6 +1182,23 @@ dependencies = [
  "uuid",
 ]
 
+[[package]]
+name = "rutster-brain-realtime"
+version = "0.1.0"
+dependencies = [
+ "async-trait",
+ "base64",
+ "futures-util",
+ "rutster-tap",
+ "serde",
+ "serde_json",
+ "tokio",
+ "tokio-tungstenite",
+ "tracing",
+ "tracing-subscriber",
+ "url",
+]
+
 [[package]]
 name = "rutster-call-model"
 version = "0.0.0"