code-mode standalone: extract protocol and add host crate (#27724)

This is phase 1 of a 4 phase stack:
1. **Add protocol and host crates for new IPC code mode implementation**
2. Create the new standalone binary
3. Create a new IPC `CodeModeSessionProvider` to use new binary
4. Remove v8 from core and only use IPC provider


## Add protocol and host crates for new IPC code mode implementation
Establish a clean process boundary without changing the existing
in-process behavior.

- Add the codex-code-mode-protocol crate for shared session, runtime,
response, and tool-definition types.
- Move protocol-facing code out of the V8-backed implementation.
- Add a buildable codex-code-mode-host crate as the foundation for the
standalone process.
- Keep the existing in-process runtime as the active implementation.

codex-rs/code-mode/src/runtime/callbacks.rs

@@ -1,4 +1,4 @@
-use crate::response::FunctionCallOutputContentItem;
+use codex_code_mode_protocol::FunctionCallOutputContentItem;
 
 use super::EXIT_SENTINEL;
 use super::RuntimeEvent;