ci: auto-format AI changelog + bump actions to v6

[AndreaBonn]

Summary

Two related CI fixes bundled together:

1. Auto-format generated CHANGELOG — AndreaBonn/ai-changelog-generator@v1 emits Markdown without blank lines after ### Headings, breaking format:check. Added a post-step that runs prettier --write CHANGELOG.md and commits the fix with [skip ci].
2. Bump actions to v6 — actions/checkout@v4 and actions/setup-node@v4 run on Node.js 20, deprecated by GitHub Actions starting 2026-06-16. Bumped to @v6 (Node.js 24 runtime) across all 4 workflow files.

Why

• Fix 1: every release would otherwise require a manual prettier --write CHANGELOG.md + commit to unbreak CI (happened on v1.3.1).
• Fix 2: pre-emptive — Node 20 is removed from runners on 2026-09-16; v6 is the current stable.

Test plan

• CI green on PR (uses bumped actions immediately)
• Next release triggers AI Changelog workflow and produces prettier-compliant CHANGELOG without manual intervention