Add full Grok Build tool-activity compatibility

[CompleteDotTech]

Beads: cave-trt.5

Extend the direct Grok Build implementation in src/lib/grok-build.ts and handleGrokLine to surface tool lifecycle events, not only text/end/error. Grok is not installed on the development host. Probe grok --version and help, then capture event sequences for supported streaming-json versions; official overview: https://docs.x.ai/build/overview.

Shared acceptance criteria

• Full live and persisted tool activity: running/completed/error, stable IDs, inputs/outputs, resume, and accessible status.
• Discover the local runtime version and capabilities; resolve only a validated signed registry schema with offline cache and rollback protection.
• Support concurrent schema versions and compatible schema refreshes without a Cave UI release.
• On unsupported versions or unknown events, retain plain chat and show a visible diagnostic; do not silently lose tool activity or log sensitive payloads.
• Cover every supported schema version and fallback with fixture-based conformance tests; preserve backward compatibility.