Gateway slash command responses were silently dropped — use-socket.ts
had no handlers for command:result or system:reload. Added both:
- command:result → renders success/error as a system message
- system:reload → updates command registry manifest + shows reload message
Also removes stray @testing-library/react devDependency that was
never used in the CLI package.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>