Guard default repetition penalty flag for older external engines

[Rishirandhawa]

Fixes #75

Summary

• detect the resolved local engine version before building session launch args
• omit --default-repetition-penalty when the app is launching an older external vmlx-engine that predates that CLI flag
• log a clear upgrade hint instead of hard-failing session startup
• keep bundled/project-venv engines on the current behavior
• update the settings-flow test mirror to cover the real default repetition penalty and the legacy-engine compatibility path

Verification

• npm test -- settings-flow.test.ts
• npm run typecheck (currently still fails on the pre-existing renderer typing error at panel/src/renderer/src/components/sessions/SessionConfigForm.tsx:937)