refactor: split release into two scripts for safer workflow
- release-notes.sh: generates build/release-notes.md from commits since
last tag via LLM, without bumping version or creating tags
- release.sh: requires release-notes.md to exist, refuses without it.
Skips bump if tag already exists (safe re-run). Deletes and recreates
GitHub Release if it already exists (idempotent).
- Update /release skill to match the two-step flow
This prevents the previous failure mode where a monolithic script would
run bump+tag+build before notes could be reviewed.