Add a release note

A user-visible change adds a fragment to the changelog of the program it changes, in the pull request that earns it:

Every release ships both programs under one version. The desktop embeds its changelog and shows the fragments as the draft in the app; the CLI's notes ship on the GitHub release. Either way, what you write here is the product's changelog, not a note to a future maintainer.

A change to the shared internal/ packages earns a note in each program whose users can notice it, and only there. One that changes both programs' behaviour gets one fragment per program, each written for that program's users.