The second phase of the `/folder-explore` → `/folder-plan` → `/folder-build` → `/folder-log` loop. Produces a structured six-section plan in chat the user reviews before `/folder-build` executes. **Chat-first**: it writes no durable stub. A plan changes nothing on disk, so there's nothing for the folder's history layer to record yet (single-source history — one layer per folder, and the plan isn't an event in it); the durable in-flight signal is delegated instead (Step 4).
