feat: add environment-based local auth mode for ai-delegate

Add `isLocalAuthEnabled()` utility function that controls the
`allowLocalMode` option for ai-delegate initialization:

- Explicit override via NEXT_PUBLIC_AI_DELEGATE_LOCAL_AUTH env var
- Defaults to development mode (NODE_ENV === 'development')
- Adds console logging showing local auth mode status

This allows local auth mode to be disabled in development or
enabled in production as needed via environment configuration.

Closes #136

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>