Add xAI Grok Build (`grok`) as a first-class Multica runtime over ACP
(`grok --no-auto-update agent --always-approve stdio`), reusing
hermesClient like traecli and kimi. Includes daemon discovery,
protocol_family migration 174, model discovery, MCP passthrough,
thinking effort, frontend branding, and product docs.
Follows xAI's documented headless ACP flow: after `initialize`, read the
advertised `authMethods` and send `authenticate` (preferring xai.api_key
when XAI_API_KEY is set, else the cached login token) before any session
operation — a real, logged-in CLI rejects session/new and session/load
without it. Model discovery performs the same handshake so it returns the
live catalog instead of the static fallback.
`--no-auto-update` is passed as a global flag (and kept daemon-owned in
the blocked custom-arg set) so a background update check can't stall an
unattended ACP task. Thinking effort uses the current `--effort` flag,
and the minimum grok version is 0.2.89 (ACP + authenticate + session/load
+ session/set_model + MCP + --effort).
Closes#2895
