feat: add Grok as a built-in launch preset

[sterling-crispin-at-eichler]

Grok currently requires entering a custom command for each new session. Add a built-in Grok preset that launches grok and is available in the new-session menu and settings.

Bump the settings persistence version so existing installations receive the preset, preserving edited commands, custom presets, and the selected default. Migration keeps saved presets when adding a built-in takes the list beyond the limit; the limit still applies to new additions.

Validation

• bun run lint
• bun run typecheck
• bun run test — 1,093 tests passed
• bun run build
• CI=1 bun run test:e2e tests/e2e/grok-preset.spec.ts — verifies selection and the literal grok launch request, with the request intercepted to avoid requiring an installed Grok CLI
• Migration regressions cover existing settings, customized commands/defaults, lists at and above the preset limit, and duplicate prevention. Legacy migration tests now exercise the actual persisted store.