feat(computer): add Grok Build as a BYOA engine (#23)

* feat(computer): add Grok Build as a BYOA engine

Pairing a computer can now select grok alongside claude and codex. The
daemon drives `grok agent` over ACP stdio (with a one-shot `grok -p`
fallback), seeds AGENTS.md like the other adapters, and records hops as
byoa-grok. Same-turn steer is a no-op on ACP; the next wake picks up the
ping.

* fix(test): satisfy EnginePersona.systemPrompt in grok seedHome test

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Co-authored-by: yetone <yetoneful@gmail.com>