ci(workflows): improve maintain-milestones docs and due-date handling

[kopardev]

Changes

Summary

• add comprehensive inline workflow documentation to maintain-milestones.yml
• clarify managed milestone scope, triggers, inputs, permissions, and action types
• change milestone due_on payload from T00:00:00Z to T12:00:00Z to avoid previous-day rendering in western time zones

Validation

• verified workflow syntax changes in .github/workflows/maintain-milestones.yml
• commit pushed to create_archive_milestones

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Issues

PR Checklist

(Strikethrough any points that are not applicable.)

• This comment contains a description of changes with justifications, with any relevant issues linked.
• Write unit tests for any new features, bug fixes, or other code changes.
• Update docs if there are any API changes.
• Update CHANGELOG.md with a short description of any user-facing changes and reference the PR number. Guidelines: https://keepachangelog.com/en/1.1.0/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.20%. Comparing base (e178230) to head (096ad81).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #127   +/-   ##
=======================================
  Coverage   85.20%   85.20%           
=======================================
  Files           9        9           
  Lines         311      311           
=======================================
  Hits          265      265           
  Misses         46       46           

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