* docs: add changeset guidelines to contributing docs
Contributors and agents need to know when and how to add changesets.
Without one, changes to published packages won't trigger a release.
- CONTRIBUTING.md: full Changesets section with when/when-not, how to
add, writing descriptions, examples for patch/minor/major
- AGENTS.md: concise changeset subsection using --empty for
non-interactive use, added as step in PR flow
- PR template: changeset checkbox in checklist
* docs: note that multiple changes need separate changesets
0aae9d806b