feat(commands): BTW side-session command (spec)

[nori-sessions]

Summary

• Adds APPLICATION-SPEC.md and CURRENT-PROGRESS.md for the /btw side-session feature
• /btw <question> creates a secondary ACP session on the same NDJSON wire to answer one-off questions without interrupting the primary session
• Uses wire-level conversation history capture for agent-agnostic context sharing (no transcript-from-disk, no session/fork dependency)
• Covers: protocol changes (Op::Btw, BtwStarted/Delta/Complete/Error events), backend (history capture, prompt builder, side-session event routing, lifecycle handler), and minimal TUI rendering

Key design decisions

1. Wire capture over transcript-from-disk — general to any ACP agent
2. Same-wire multiplexing — SacpConnection routes events by sessionId to separate channels
3. Inline TUI rendering — BTW cells appear in chat history with a "BTW" label, no separate panel
4. 5-minute timeout, one BTW at a time — simple resource management for v1

Test plan

• Review spec and approve implementation approach
• Implementation follows TDD — tests written before code