feat: add GitHub Actions CI and perf:ignore comment support

[cschuman]

Summary

• Add comprehensive GitHub Actions CI workflow
• Add // perf:ignore comment support for suppressing false positives

CI/CD

• Multi-version Go testing (1.21, 1.22, 1.23)
• golangci-lint integration
• Self-audit (dogfooding goperf on itself)
• Cross-platform binary builds (Linux, macOS, Windows)

Ignore Comments

// perf:ignore           - Ignore next line
// perf:ignore sql-in-loop - Ignore specific rule
// perf:ignore-start     - Start ignore block
// perf:ignore-end       - End ignore block

Test Plan

• All 6 ignore tests pass
• Build succeeds
• Self-audit passes