ci: post only the merge commit trailers and read release props from them

[josephfusco]

Drops the Core SVN section #66 added by going back to format: git, and teaches the release props aggregator to read WordPress.org slugs from the Co-authored-by trailers, which it never did before — under the git format it credited only contributors it recovered through the unlinked lookup.

Use of AI Tools

Claude Code was used to make this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.94%. Comparing base (620d94d) to head (e6f69f4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main      #68   +/-   ##
=========================================
  Coverage     56.94%   56.94%           
  Complexity       42       42           
=========================================
  Files            11       11           
  Lines           367      367           
=========================================
  Hits            209      209           
  Misses          158      158           

Flag	Coverage Δ
multisite	56.94% <ø> (ø)
phpunit	52.47% <ø> (ø)

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[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core SVN

Core Committers: Use this line as a base for the props when committing in SVN:

Props joefusco.

GitHub Merge commits

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: josephfusco <joefusco@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.