#456 chore: adopt changelog fragments so PRs stop colliding on CHANGELOG.md

[Anonymous]

Originally created by: fu351

Every PR currently inserts its bullet at the top of ## Unreleased, so any two open PRs conflict with each other and with every maintainer merge — this week one dependency pin (#452) conflicted six open PRs at once. The standard fix is changelog fragments (towncrier or the same pattern hand-rolled): each PR adds its own file under changelog.d/<pr-number>.md, and a release step compiles the fragments into CHANGELOG.md in one commit. Scope: add the directory + a short convention note in CONTRIBUTING, wire the compile step into the release checklist (RELEASING.md), and migrate nothing retroactively.

[Anonymous]

Ticket changed by: fu351

• status: open --> closed