Add workflow to sync PR labels from closing issues

[drbenvincent]

Introduces a GitHub Actions workflow that synchronizes labels from issues closed by a pull request, excluding 'help wanted' and 'good first issue' labels. This helps maintain label consistency between issues and pull requests.

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📚 Documentation preview 📚: https://causalpy--601.org.readthedocs.build/en/601/

[Copilot]

Pull request overview

This PR adds a GitHub Actions workflow that automatically synchronizes labels from issues to pull requests that close those issues. The workflow uses the williambdean/closing-labels action and excludes common triage labels ('help wanted' and 'good first issue') from being copied.

Key Changes

• Introduces automated label synchronization from closing issues to PRs
• Uses pull_request_target trigger for security with appropriate write permissions
• Configures exclusion of triage labels to maintain clean PR labeling

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.27%. Comparing base (2d6bba7) to head (e77a373).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #601   +/-   ##
=======================================
  Coverage   93.27%   93.27%           
=======================================
  Files          37       37           
  Lines        5632     5632           
  Branches      367      367           
=======================================
  Hits         5253     5253           
  Misses        248      248           
  Partials      131      131           

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