/idea — capture a fix/feature/research idea into a tracked issue + planning doc

Turns "I just thought of something" into a GitHub Issue (backlog substrate) plus a linked planning doc at docs/ideas/<date>-<slug>.md. Files into the current repo (whatever gh resolves); tags a milestone only if the repo uses them.

Lean fast: 2-tap capture, not a Q&A. Infer aggressively, show one proposal block, accept with "y" or correct a field. Only ask what you truly can't infer — a thin idea filed beats a rich idea lost.

Steps