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-host/BUILD.bazel

@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "code-mode-host",
+    crate_name = "codex_code_mode_host",
+)

codex-rs/code-mode-host/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "codex-code-mode-host"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[[bin]]
+name = "codex-code-mode-host"
+path = "src/main.rs"
+
+[lints]
+workspace = true

codex-rs/code-mode-host/src/main.rs

@@ -0,0 +1 @@
+fn main() {}