agent/codex: split websocket protocol, wire approvals, and harden tests

[jlewi]

Summary

Implements the backend side of the Codex app-server integration in runme, including protocol separation, execution approval wiring, and notebook/session bootstrap behavior from the design.

What changed

• Added a dedicated Codex websocket protocol package and generated artifacts under api/proto/agent/codex/v1 (kept separate from cell-execution websocket APIs).
• Wired Codex turn execution and MCP execution approval handling in the agent server path.
• Added session/bootstrap plumbing for Codex app flows.
• Hardened tests for environment/tooling variability and shell startup noise:
  • runtime-gated tests for deno, rust-script, and working bb
  • conditional direnv expectations
  • robust TTY output assertions that strip known shell init noise
  • stabilized txtar and autoconfig tests for local/dev environments

Validation

• runme run lint test

Related

• Use codex app as the agent harness web#34