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/Cargo.lock

@@ -2479,10 +2479,10 @@ dependencies = [
 name = "codex-code-mode"
 version = "0.0.0"
 dependencies = [
+ "codex-code-mode-protocol",
  "codex-protocol",
  "deno_core_icudata",
  "pretty_assertions",
- "serde",
  "serde_json",
  "tokio",
  "tokio-util",
@@ -2490,6 +2490,22 @@ dependencies = [
  "v8",
 ]
 
+[[package]]
+name = "codex-code-mode-host"
+version = "0.0.0"
+
+[[package]]
+name = "codex-code-mode-protocol"
+version = "0.0.0"
+dependencies = [
+ "codex-protocol",
+ "pretty_assertions",
+ "serde",
+ "serde_json",
+ "tokio",
+ "tokio-util",
+]
+
 [[package]]
 name = "codex-collaboration-mode-templates"
 version = "0.0.0"