feat: standalone CTA Transitions workflow

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: Use a standalone CI file for CTA transitions #2009
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Moves the transition mode logic into two files:

1. .github/actions/transition/action.yml: A dedicated composite action for running in transition mode and checking in & commenting any changes
2. .github/workflows/cta.yml: Checks if the action should be run, and runs it if so

This means the job won't be marked as Skipped if the logic is skipped. It'll still be Success and thus not block PRs.

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.80%. Comparing base (cb898c9) to head (67c2c96).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2012   +/-   ##
=======================================
  Coverage   99.80%   99.80%           
=======================================
  Files         113      114    +1     
  Lines        4573     4616   +43     
  Branches      285      287    +2     
=======================================
+ Hits         4564     4607   +43     
  Misses          9        9           

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[github-actions]

🎉 This is included in version v2.10.0 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀