feat(agents): add Grok Build as a native ACP runtime

[daniel-farina]

Summary

Closes #2347.

• Registers Grok Build in Buzz's managed ACP runtime catalog (id: grok) so Desktop can discover and spawn it like Goose/Claude/Codex.
• Uses Grok's native ACP entrypoint (grok agent --always-approve stdio) — no separate *-acp npm adapter.
• Adds ~/.grok/bin to binary discovery, process-sweep name grok, onboarding visibility, docs, and unit tests for default arg normalization.

Why

Grok Build already speaks ACP over stdio (grok agent stdio). Users currently cannot select it as a first-class Buzz harness without a custom command. This mirrors the Pi/Cursor runtime registration pattern.

What changed

Area	Change
KNOWN_ACP_RUNTIMES	New Grok Build entry
Default args	agent,--always-approve,stdio (also maps bare legacy acp → runtime default)
PATH	Discovers ~/.grok/bin
Onboarding	Adds grok to runtime order + readiness CLI path
Docs	README / TESTING / buzz-acp README
Tests	normalizes_grok_args_to_agent_stdio (buzz-acp + desktop)

Validation

• cargo test -p buzz-acp --lib normalizes_ — 6 passed (incl. Grok)
• Live ACP handshake: BUZZ_ACP_AGENT_COMMAND=grok buzz-acp models lists Grok models
• Live harness against hosted relay: buzz-acp + Grok connected as Fizz, presence online, channel reply E2E
• Desktop full rebuild / Playwright (sidecar build path not fully exercised in CI notes)

How to try

curl -fsSL https://x.ai/cli/install.sh | bash
grok login   # or export XAI_API_KEY=...

export BUZZ_PRIVATE_KEY=nsec1...
export BUZZ_RELAY_URL=wss://your-relay
export BUZZ_ACP_AGENT_COMMAND=grok
buzz-acp

Then @mention the agent from Buzz Desktop (owner-only by default).

Checklist

• New behavior has tests
• User-facing setup is documented
• No new unwrap() in production code
• No unsafe code

[daniel-farina]

Tracking issue: #2347