feat(desktop): register Grok Build as native ACP runtime + onboarding harness

[Bartok9]

Summary

Register Grok Build as a first-class ACP harness and show it on Set up your agent harnesses next to Claude Code and Codex.

Closes #2347. Companion: Cursor harness #2536 / #2535.

Why

Grok Build already speaks ACP natively:

grok agent stdio
# managed default:
grok agent --always-approve stdio

Without catalog registration, Desktop cannot discover, install-hint, or process-sweep Grok the way it does other runtimes. Onboarding also hard-filters visible harnesses (ONBOARDING_RUNTIME_ORDER).

What changed

• KNOWN_ACP_RUNTIMES: grok entry (commands: grok, aliases grok-build / grokbuild)
• Default args: agent --always-approve stdio (avoids TTY tool-permission blocks for managed agents)
• PATH discovery includes ~/.grok/bin
• Skills: .grok/skills
• Auth: login_hint for grok login / XAI_API_KEY (no stable auth status probe in current CLI — auth_probe_args: None, same posture class as Goose)
• Onboarding visibility: claude, codex, grok
• agentReadiness treats Grok like other CLI harnesses when available
• Process-sweep binary name grok
• Unit tests + buzz-acp README

Non-claims

• Does not add Cursor (separate PR feat(desktop): register Cursor as native ACP runtime + onboarding harness #2536).
• Does not invent a fake grok auth status probe — CLI currently has grok login / grok auth setup, not a codex-style status subcommand.

Test plan

• cargo test --lib normalizes_grok
• cargo test --lib resolves_grok
• cargo test --lib grok_runtime
• node --test onboardingRuntimeSelection.test.mjs
• Desktop: harness page shows Grok Install; after install + grok login / XAI_API_KEY, agent spawn uses grok agent --always-approve stdio

Reviewer notes

• --always-approve is intentional for managed agents (Buzz spawns without a TTY). Users who want interactive approval can override agent args.
• Onboarding allowlist remains product-scoped (not goose/buzz-agent).

Signed-off-by: Bartok9 danielrpike9@gmail.com

[Bartok9]

Coordination note with Cursor PR #2536

Grok Build installs both grok and agent PATH shims (~/.grok/bin/agent, often linked as ~/.local/bin/agent).

Cursor’s PR intentionally does not treat basename agent as Cursor — it prefers cursor-agent. Please keep these PRs consistent if either is revised:

• Grok: spawn via grok + args agent --always-approve stdio
• Cursor: spawn via cursor-agent + args acp

[Bartok9]

Polish pass (rebased on upstream/main)

HEAD ea75f3e6 — rebase + maintainer docs/tests.

Differentiation vs Cursor (#2536) and other Grok PRs

• Binary: registry commands: &["grok"] only — ACP via args agent --always-approve stdio.
• Never claims bare PATH shim agent (that disambiguation stays on Cursor’s own entry).
• Onboarding: ONBOARDING_RUNTIME_ORDER includes grok.
• Docs/tests: discovery comment + README maintainer note + grok_does_not_claim_bare_agent_path_shim unit test.

Test plan

• Discovery unit tests for grok args normalization + bare-agent non-claim (run in CI with desktop mesh-llm as configured by repo).