POST …/plan answers which opened issues will not start yet and why: a hot file, a pin or an open pull #7427

Intent: a planner sees at once which of its issues are held, and can rewrite them in the same run instead of finding out from the map later.

Done:

• [ ] The plan answer (built in src/actions/plan.ts) gains held: [{ key, number, by, detail }] for each opened issue that would not start now: a file it names on its Files: line is hot (hotOverlap), a pinned file holds it, or an open pull already proposes it. It uses the same words as the map's held.detail.
• [ ] ready and later stay as they are, and an issue that can start is not listed in held.
• [ ] docs/PLANNING.md names the field in one line.
• [ ] A test (e.g. test/plan-held.test.ts) makes a file hot with remembered pull files, posts a plan with one item naming it and one not, and checks that only the first is in held with the map's sentence.

Files: src/actions/plan.ts, docs/PLANNING.md, a new test.