Harden GitHub workflows

[samdark]

Summary

• pin third-party GitHub Actions and reusable workflow refs to immutable SHAs
• add explicit workflow permissions
• disable checkout credential persistence where applicable

Testing

• zizmor --no-progress --no-exit-codes .github/workflows
• git diff --check

[Copilot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.29%. Comparing base (fe63447) to head (972f7be).
⚠️ Report is 109 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #806      +/-   ##
============================================
+ Coverage     94.40%   96.29%   +1.89%     
- Complexity      953     1154     +201     
============================================
  Files           108      124      +16     
  Lines          3018     3540     +522     
============================================
+ Hits           2849     3409     +560     
+ Misses          169      131      -38     

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