Add interactive multi-turn chat REPL (mmx text repl)

[NianJiuZst]

Summary

Currently mmx text chat is single-turn — each invocation requires manually constructing the full message history via --message flags or --messages-file. This is cumbersome for iterative conversations.

Proposal

Add mmx text repl — an interactive multi-turn chat REPL with:

• Automatic conversation context management across turns
• Real-time slash command suggestions (type / to trigger, filters as you type)
• Custom raw-mode line editor with a bordered input area
• 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)

Implementation

See linked PR.