feat: add /btw side-question slash command

[gsabatie]

Summary

Adds /btw <question>, a one-shot side question you can ask between turns,
using the current system prompt and session history as read-only context,
with no tools. Neither the question nor the answer is added to the main
conversation history — the next model turn sends the same history as before
/btw. Only available while the agent is idle (blocked while a turn is
running); interactive mode only, -p and ACP reply with an interactive-only
message.

Demo

Design choices

• Builds a local message list ([*self.messages, question]) and calls the
  backend directly with tools=[], so the model can't trigger tool calls
  during the side question. Nothing is appended to self.messages.
• Token usage is still counted in session stats (/status) via the normal
  accounting path.
• UI follows existing Mistral patterns: slash echo + ExpandingBorder aside
  (BtwMessage), with the usual Generating spinner during the call.

Limitations

• Session meta.json stats only rewrite when messages change; /status is
  live in-memory.
• Not available in programmatic (-p) or ACP surfaces by design.
• Blocked while the agent is actively running a turn.

[ED-RO]

💯 👌👌