Add native Grok ACP provider

[jsgrrchg]

Summary

• add Grok as a native ACP provider with setup, auth, runtime metadata, and model/config handling
• preserve ACP-declared controls and slash commands without synthetic Grok defaults
• harden Grok review isolation, tab icon, docs, and smoke/test coverage

Tests

• cargo test -p neverwrite-native-backend
• npm test -- src/features/ai/components/AIChatComposer.test.tsx src/features/ai/components/AIChatAgentControls.test.tsx
• npm test -- src/features/editor/editorTabIcons.test.tsx
• npm test -- src/features/settings/AIProvidersSettings.test.tsx src/features/ai/store/chatStore.test.ts -- --runNamePattern "default runtime|Default agent|explicit default|falls back when a persisted default|keeps current runtime|uses the explicit default runtime"
• npx tsc -b --pretty false
• git diff --check

Closes #183