Add reusable release workflows

[tvdeyen]

Adds the shared release workflows from AlchemyCMS/.github:

• prepare-release.yml - Manually trigger to create a release PR
• release.yml - Publishes to RubyGems when release PR is merged
• post-release.yml - Bumps to next dev version after release + announces to Slack, Mastodon, and Bluesky

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.19%. Comparing base (c7e6dca) to head (b387634).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3510   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files         287      287           
  Lines        7558     7558           
=======================================
  Hits         7346     7346           
  Misses        212      212           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[alchemycms-bot]

💚 All backports created successfully

Status	Branch	Result
✅	8.0-stable

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details