ci: automate release notes generation

Part of #715

### Problem

Release notes are currently written manually by reviewing the git log between tags. This is time-consuming, error-prone (easy to miss PRs), and inconsistent in format.

### Solution

Adopt a changelog fragment approach (similar to Towncrier or git-cliff):

1. Each PR that should appear in release notes includes a changelog fragment file
2. At release time, fragments are assembled into a structured changelog
3. Fragments are categorized by type (feature, bugfix, improvement, breaking change, etc.)

### Options

- **Towncrier**: Python-based, well-established. Uses fragment files in a \`newsfragments/\` directory. Each fragment is named \`{issue_number}.{type}.rst\`.
- **git-cliff**: Rust-based, generates changelogs from conventional commits. No fragment files needed but requires commit message discipline.
- **GitHub release notes**: Built-in, generates from PR titles and labels. Least effort but least control.

### Recommendation

Start with GitHub's built-in release notes generator configured via \`.github/release.yml\`, using our existing label taxonomy (\`kind/*\`) to categorize entries. This is zero-friction for contributors. Move to Towncrier later if more control is needed.

### Acceptance criteria

- [ ] \`.github/release.yml\` configured with label-to-category mapping
- [ ] Release notes generated automatically for each tag
- [ ] Breaking changes highlighted in their own section