Retire the changelog-refresh cron: release-plz is the single changelog writer

[RAprogramm]

Two automations write to CHANGELOG.md:

• the weekly changelog-refresh.yml cron regenerates ## [Unreleased] with git-cliff and opens a PR from GITHUB_TOKEN (so CI never runs on it — see the note in its own PR body; chore(changelog): auto-refresh Unreleased section #256 has been open since 2026-07-27);
• release-plz opens and continuously updates the release PR (chore: release v0.12.6 #257) with the version bump and a curated ## [x.y.z] section built from the same commits.

The second writer adds nothing the release PR does not already show and causes real damage: the [0.12.5] section now contains the same ### CI block twice (one release-plz entry, one cron entry with a dependabot avatar), and whichever PR merges first makes the other conflict. Keep a Changelog also warns against commit-log dumps as changelog content, which is exactly what the cron produces.

Fix, following the single release-PR model used by release-plz / release-please:

• delete .github/workflows/changelog-refresh.yml
• drop the stale reference to it in the Changelog job comment in ci.yml
• remove the duplicated cron-generated ### CI block from the [0.12.5] section
• close chore(changelog): auto-refresh Unreleased section #256 without merging; chore: release v0.12.6 #257 stays the only release PR