Discovered by actually running `knope --validate` locally: knope treats every .md file in .changeset/ as a changeset to consume, with no special-casing of README.md by name the way the JS changesets CLI has, so a frontmatter-less README failed validation outright ('missing front matter'). Moved the convention doc to CHANGESETS.md at the repo root and updated every reference (CLAUDE.md, changeset-check.yml). Also documents a second thing the same dry-run turned up: a changeset body written as hard-wrapped multi-line prose renders with its first line as a heading and the rest as an orphaned paragraph in CHANGELOG.md — keep a body to one unwrapped line.