chore: bump GitHub Actions to node24-compatible versions

[fcostaoliveira]

GitHub is deprecating the node20 runner for GitHub Actions on June 16, 2026. Workflow steps using node20-based action versions will fail after this date.

This PR bumps all affected actions to their latest node24-compatible versions.

Note: Initial review flagged actions/checkout under-bumped to @v4; corrected to @v6 before opening.

Reviewed by 3 independent agents (opus/sonnet/haiku) before opening.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.53%. Comparing base (a4f132a) to head (2a3a5ba).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #544      +/-   ##
==========================================
+ Coverage   47.50%   47.53%   +0.03%     
==========================================
  Files          74       74              
  Lines        8907     8907              
==========================================
+ Hits         4231     4234       +3     
+ Misses       4676     4673       -3     

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