Add moderation workflow

[martincostello]

Trial the use of github/ai-moderator to detect spam.

[Copilot]

Pull Request Overview

This PR adds a GitHub Actions workflow to automatically detect and moderate spam content using the github/ai-moderator action. The workflow is configured to trigger on new issues, issue comments, and pull request review comments.

• Introduces automated spam detection for issues and comments
• Configures AI-based moderation with labeling and comment minimization
• Sets up appropriate permissions for the moderation workflow

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.12%. Comparing base (43ef928) to head (a6e2482).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2725   +/-   ##
=======================================
  Coverage   96.12%   96.12%           
=======================================
  Files         309      309           
  Lines        7118     7118           
  Branches     1008     1008           
=======================================
  Hits         6842     6842           
  Misses        222      222           
  Partials       54       54           

Flag	Coverage Δ
linux	96.12% <ø> (ø)
macos	96.12% <ø> (ø)
windows	96.11% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

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

[martincostello]

Workaround for rhysd/actionlint#548.