feat: add interactive multi-turn chat REPL command

Add `mmx text repl` — a full interactive REPL with:
- Multi-turn conversation with automatic context management
- Custom raw-mode line editor with visual chrome (bordered input area)
- Real-time slash command suggestions (type / to trigger, filters as you type)
- Slash commands: /exit, /clear, /system, /model, /save, /help, /history
- Arrow key history navigation, Tab completion, standard readline keybindings
- Streaming response display with thinking/response separation
- Graceful SIGINT handling (double Ctrl+C to exit)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: NianJiuZst