Fix Grok CLI authentication persistence

[jsgrrchg]

Summary

• detect the current ~/.grok/auth.json credential store while preserving the legacy directory fallback
• accept Grok CLI credentials renewed after a persisted authentication invalidation
• reconcile stale invalidation state across native and legacy runtime settings during startup
• isolate Grok authentication tests from the developer machine and cover restart-related regressions

Validation

• pnpm exec vitest run src/main/ai/grok/setup.test.ts src/main/ai/service.grok.test.ts
• cargo test -p comando-ai --lib
• cargo test -p comando-native-backend --lib
• pnpm run typecheck
• targeted ESLint checks
• cargo fmt --all -- --check
• git diff --check