Idea — capture an idea before it evaporates

Quickly capture an idea about the current project into docs/IDEA.md — a "what's next" backlog. Ideas can be anything project-related, technical or not, horizontal (tooling, process, docs) or vertical (a feature). One dated entry with just enough context that a future session or human can pick it up cold. Also handles pruning ("/idea unbloat" — interactive curation where surviving ideas earn a survived ×N marker) and prioritizing ("/idea sort" — an interactive triage session where the USER decides priority buckets, seeded by the survival counter, and the file is rewritten in that order). Use when the user says "/idea", "capture this idea", "note this idea", "remember this …