fix: make Grok launch availability truthful (#3861)

[CompleteDotTech]

Summary

Implements the remaining Grok Build launch-integrity gaps for #3861 on top of the shared availability contract from #3891.

• Gates Grok version and authenticated model-catalog probes on the same ready availability result used by chat.
• Treats synchronous and asynchronous spawn() failures as structured runtime errors. A post-preflight ENOENT retains runtime_missing; other launch failures use runtime_launch_failed.
• Failed launches complete through done(isError) without exposing the resolved executable or GROK_BIN path, emitting a fabricated assistant response, or persisting a fabricated assistant turn.
• Keeps runtime status, Familiar Studio, Familiar Summoning Circle, capability overview, and chat error states truthful for discovered-but-unlaunchable launchers.
• Keeps native-Windows migration fixtures capability-aware: privileged real-symlink cases run where supported while fallback coverage remains active.

Closes #3861.

Acceptance matrix

• Missing Grok emits runtime_missing before spawn, creates no child process, and never fabricates an authentication/no-output assistant reply.
• GROK_BIN discovery remains validated and invalid overrides fall through without exposing their value.
• Native Windows executables, npm shims, and WSL launcher behavior retain focused coverage.
• Grok JSONL provider errors remain actual CLI errors after a successful start.
• Status and chat share availability semantics; Grok version/model probes run only for ready plans with the same scoped environment.
• Async ENOENT and injected synchronous UNKNOWN post-preflight spawn failures complete with structured errors and cannot reach the generic authentication fallback.
• Setup, familiar selection, Summoning Circle, and capability overview distinguish unavailable from installed runtimes using shared remediation; reports without availability remain neutral.

Verification

Rebased safely onto current origin/main (38b4c0ba); head is 3db3df6d.

Passed on native Windows:

• pnpm typecheck
• pnpm lint
• pnpm check:tests-wired (1254 wired)
• node --require ./scripts/css-source-contract-hook.cjs --experimental-strip-types src/lib/runtime-availability.test.ts
• node --require ./scripts/css-source-contract-hook.cjs --experimental-strip-types src/lib/grok-bin.test.ts
• node --require ./scripts/css-source-contract-hook.cjs --experimental-strip-types src/lib/grok-build.test.ts
• node --require ./scripts/css-source-contract-hook.cjs --experimental-strip-types src/app/api/harnesses/route.test.ts
• node --require ./scripts/css-source-contract-hook.cjs --experimental-strip-types --import ./scripts/test-alias-register.mjs src/app/api/chat/send/route-runtime-availability.integration.test.ts (missing Grok zero-spawn, async ENOENT, and synchronous spawn failure)
• node --require ./scripts/css-source-contract-hook.cjs --experimental-strip-types src/app/api/api-contracts.test.ts (238 route contracts)
• Focused Familiar Studio, Summoning Circle, onboarding, and capability overview suites.

pnpm test:api was re-run after the Windows fixture guards. It intentionally skips scripts/dev-app-teardown.test.mjs on native Windows because Node cannot deliver POSIX signals to exercise Bash traps; it passes the native Windows migration suites and consistently stops only in unchanged src/app/api/beads/route.test.ts cleanup with EBUSY removing its temporary project-a Git fixture after 24 assertions. The test blob is identical to origin/main; this reproducible Windows cleanup defect remains tracked as cave-pp2 and is outside this PR. The earlier fake-server timeout was investigated: this native Windows host has only the WSL bash.exe launcher with no installed Linux /bin/bash, so the POSIX Bash/signal fixture cannot start its fake server. The native-Windows skip is environmental; scripts/dev-app.test.mjs still pins the Windows cmd.exe launch configuration. Focused Grok coverage and all static gates pass.

CI

Fresh CI and CodeQL will run for 3db3df6d after the force-push. The Swift audit is intentionally skipped.