Changelog writer

Keep CHANGELOG.md current by folding merged pull requests into the Unreleased section, following the Keep-a-Changelog convention.

Steps

1. Read the merges since the last tag: git log <last-tag>..HEAD --merges.
2. Classify each into Added / Changed / Fixed / Removed from its PR label.
3. Append entries under ## [Unreleased], newest first.

On release

Rename Unreleased to the new version + date and open a fresh Unreleased block. Section headings are defined in config/sections.json.