Bundle the diff-scoped content checks into one reusable workflow #1367

Each notes repo carries one caller file per check (for example check-note-sheet-rows.yml, check-restated-results.yml, check-one-div-per-subfile.yml). A new check such as check-div-ids (#1366) reaches a repo only after a PR adds its caller there, and a check nobody calls does not enforce anything.

Proposal: add one reusable workflow, for example check-notes.yml, that calls every diff-scoped, warn-only content check as a job. A notes repo then has a single caller, and a new check reaches every repo when the tag slides.

Open questions:

• Inputs to pass through per check (paths-ignore, fail, globs).
• Whether a failure of one check should keep the others running (it should).
• How a repo opts out of one check.

Requested by Ezra in the project thread that added check-div-ids.