📝 Require a changelog note before finishing a branch

A merged branch must carry a final commit adding a short summary of the
work under [Unreleased] in CHANGELOG.md. create:release derives the bump
heuristic from that body, so notes have to exist before release day;
rationale in development/workflow.md.

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- require a short summary under `[Unreleased]` in the changelog before a branch is finished
+
 ## [0.1.6] - 2026-09-15
 
 - restructure the documentation: README.md for users, CONTRIBUTING.md for contributors, and development/ for the decisions, rejected alternatives and known issues