Grok Build ACP adapter support

[imshunsuke]

Grok Build is the open-source coding agent from xAI. It exposes an ACP stdio endpoint through grok agent stdio, so I would like to add it as a built-in CodeCompanion ACP adapter.

Unlike the simpler ACP adapters, Grok Build sends vendor extension requests back to the client for:

• x.ai/ask_user_question
• x.ai/exit_plan_mode

CodeCompanion currently has fixed dispatch for standard ACP requests, so a functional integration also needs a small adapter-level hook for agent-to-client extension requests.

Proposed scope:

• Add a built-in grok adapter using the official command.
• Handle Grok interactive questions and plan approval or revision in the existing chat question UI.
• Cancel pending interactive requests cleanly when a prompt ends or is cancelled.
• Add documentation and focused tests.
• Add no dependencies.

I have a local prototype based on the current main. The adapter and relevant ACP and question tests pass, and the full suite currently passes (1,261 cases, 0 failures).

Would this scope fit CodeCompanion? If so, I would be happy to prepare the PR.

AI usage: I used OpenAI Codex to help research the protocol, implement the prototype, and draft tests. I will disclose this in the PR and manually review the full diff before submitting.

[olimorris]

I'll take a look at some point. If it deviates too much from the protocol then I'll be leaning towards not merging and ask users to leverage the CLI interaction mode.