code-mode standalone: extract protocol and add host crate

[cconger]

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.

[chatgpt-codex-connector]

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[anp-oai]

For my own understanding: are we going to allow multiple sessions in the same OS process, or will they have process isolation from each other?

[cconger]

addressed offline.

[anp-oai]

nit: IME default trait methods often end up with surprising missing implementations, is there anything that could go wrong if a caller forgets to implement this?