Add golangci-lint

[destel]

• Add golangci-lint config and run it in CI
• Modernize code (wg.Go, range-over-int, slices.Sort; stdlib slices/cmp in helpers)
• Fix formatting and drop redundant loop-var copies

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.56%. Comparing base (0ee424d) to head (8147822).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   99.57%   99.56%   -0.01%     
==========================================
  Files          15       15              
  Lines         708      696      -12     
==========================================
- Hits          705      693      -12     
  Misses          3        3              

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