feat: automate CHANGELOG.md updates from GitHub Release notes

[Copilot]

CHANGELOG.md had no automated update mechanism — entries had to be added manually or not at all. This wires the existing gh release create --generate-notes output directly into CHANGELOG.md on every stable release.

What changed

.github/release.yml (new)

Configures GitHub's auto-generated release note categories via PR labels: breaking, feat/enhancement, fix/bug, docs/documentation. Unlabelled PRs fall into "Other Changes"; ignore-for-release suppresses an entry entirely.

CHANGELOG.md

Replaced the placeholder with a Keep a Changelog header and an HTML comment marker that the workflow targets for insertion:

<!-- New entries are prepended automatically by the release workflow. -->

.github/workflows/release.yml — update-changelog job

New job, runs after release, stable branches only (is-release == 'true'):

1. Fetches the release body: gh release view <version> --json body --jq '.body'
2. Inserts ## <version> — YYYY-MM-DD + release notes immediately after the comment marker using awk, leaving the rest of the file intact
3. Commits and pushes back to release/X.Y (same pattern as the existing promote-api-files job)

merge-to-main gains update-changelog in its needs, so the PR to main always includes the updated file.

.github/PULL_REQUEST_TEMPLATE.md

Added checklist item: apply one of the release-note label categories (breaking / feat / fix / docs).

docs/workflows.md

Added update-changelog row to the release.yml job table.