Add Grok CLI as a built-in agent

[edimitchel]

Grok CLI (xAI Grok Build) was only usable through a custom .berth/config.json agents entry, with no session-state hooks. This adds it as a built-in preset and, because Grok Build documents Claude-style hooks, installs those hooks the same way as the other agent CLIs.

Fixes #2

What changed

• Built-in preset grok / Grok CLI: command grok, first prompt as the last argument (grok "fix the bug"), --model and --effort from the CLI (none through max). Offered when grok is on the box's PATH, like the other built-ins.
• berthd integrations install grok (and auto-detect / all) writes ~/.grok/hooks/berth.json for SessionStart, UserPromptSubmit, PostToolUse, Notification, Stop, StopFailure, StopCancelled, and SessionEnd.
• Adapter maps Grok's camelCase payload (sessionId, notificationType) and treats permission_prompt as needs-you. idle_prompt and task_complete settle a turn that never reported Stop; they are not questions.
• App label, icon, Settings → Agents install hint, and hook-install offer.
• Docs: First agent, agent integrations, config, project-config, sessions-and-agents, CLI/berthd references.

Not in this PR

• Skills and MCP are not installed for Grok. Unlike Claude/Codex/Gemini, there is no documented Berth-equivalent settings path for those yet; inventing one would be guesswork. Hooks still register so working / needs-you / done work.

Tests

• Preset command line (grok --model … --effort … '…') and agentOf
• Hook install is idempotent and includes every Grok event
• Detect / install all lists include grok
• Translate: camelCase and snake_case ids, notification types, no prompt/message leakage

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