fix(tui): forward cwd changes through app-server

Forward standalone turn-context overrides from the TUI to loaded app-server threads via `thread/turnContext/update`, so `/cwd` updates the core thread state instead of only local UI state.

Add protocol schemas and regression coverage showing thread-scoped shell commands use the updated cwd after the context override.

codex-rs/app-server-protocol/schema/json/v2/ThreadTurnContextUpdateResponse.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "ThreadTurnContextUpdateResponse",
+  "type": "object"
+}