feat(grok): add fail-closed tool activity compatibility

[CompleteDotTech]

Fixes #3845. Beads: cave-trt.5.

Compatibility behavior

• Built-in Grok Build support is restricted to the source-verified streaming-json frames: text, thought, end, and error; it claims no tool lifecycle aliases.
• Tool activity is enabled only by a selected Ed25519-signed schema with exact locally probed Grok Build version declarations, stable IDs, and a launcher-proven value-bearing --output-format streaming-json capability. Probing uses the exact preflight-approved launcher and familiar-scoped environment. No tool envelope names were inferred or captured.
• Registry refresh is bounded and fail-closed: canonical signature payload, key IDs/rotation, strict schema/key/URL validation, immutable checkpoint/high-water journal, cache integrity/expiry/rollback protection, lock ownership, and in-process schema quarantine.
• Production accepts only release-packaged Grok registry trust material; mutable COVEN_GROK_SCHEMA_REGISTRY_* overrides are development-only. The five-second refresh deadline covers response headers and a stalled body.
• A client that previously accepted a newer remote contract never revives the older compiled parser when that cache expires or its high-water anchor is missing/corrupt; it remains in plain chat until a verified refresh. The compiled baseline also expires into an accessible plain-chat fallback.
• Unsupported, malformed, or unverified structured output falls back to safe plain chat with redacted accessible diagnostics. Parseable object/array payloads are dropped while actual prose beginning with [ or { is retained.
• The real-route fixture creates an ephemeral signed test schema with fixture_tool_* aliases. Those aliases are not Grok protocol claims; it verifies selected-schema routing, persistence, terminal errors, duplicate frames, and reordered progress/results.

Evidence

Verified 2026-07-26 against the xAI Grok Build overview and upstream headless-mode documentation at revision 47348d13ec4508dcfe440e34c6d511bb02998fb2. These document headless --output-format streaming-json and only text/thought/end/error frames, not tool lifecycle events. No local Grok installation, credentials, paid run, fabricated capture, or undocumented Grok event name was used.

Latest remediation and validation

• Rebased onto current origin/main at 76d74e66 and force-pushed 605596c8cf4e063366cd9a76fce88d0d7b70419a; no Grok protocol behavior changed during rebase.
• Passed focused Grok build/compatibility/real-route/harness-routing/release-guard tests, pnpm check:tests-wired, pnpm typecheck, pnpm lint, and git diff --check after the rebase.
• The prior head's CI and CodeQL were fully green. Fresh CI/CodeQL is pending on the rebased head.

Release prerequisite

A maintainer-owned release environment must provide GROK_SCHEMA_REGISTRY_URL, GROK_SCHEMA_REGISTRY_PUBLIC_KEY or GROK_SCHEMA_REGISTRY_PUBLIC_KEYS, and GROK_SCHEMA_REGISTRY_CHECKPOINT. They are public verification material; never add a private signing key.