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]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a3c5b97) to head (7729cd6).

Additional details and impacted files

@@             Coverage Diff             @@
##              master      #342   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity        46        46           
===========================================
  Files             13        13           
  Lines            659       659           
===========================================
  Hits             659       659           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.