queue: capture the thought, keep the thread

Mid-arc ideas are load-bearing and fragile: interrupt the work to chase them and you lose the arc; ignore them and you lose the idea. Queue does neither. The user says it, you write it down, you go back to what you were doing.

Capturing (the 30-second contract)

1. Append to docs/BACKLOG.md (create it if missing), newest on top, using the entry format below.
2. Keep the user's own words where possible - the phrasing carries intent.
3. If the item is a question the current session can already answer from work done this arc (research fresh in context), give a SHORT take in chat after capturing - three sentences, a recommendation, done. Do not open a research thread; if it deserves one, that is a future arc and the entry says so.
4. Return to the interrupted work in the same turn. Queue never ends a turn as the main event.