feat(go): improve go-code-review skill

[anderskev]

Summary

• Rewrites the go-code-review SKILL.md with a proper review workflow (check go.mod version → scan full functions → verify), [FILE:LINE] output format, and Go-specific severity calibration
• Adds modern Go coverage: errors.Join (1.20+), slog (1.21+), sync.OnceValue/sync.OnceFunc (1.21+), errgroup, generics anti-patterns, range-over-int (1.22+)
• Adds version-aware context-sensitive rules (loop variable capture is pre-1.22 only, slog suggestion requires 1.21+)
• Improves all 4 reference files with reasoning-based explanations over rigid rules

Test plan

• Ran 3 eval prompts (bug-heavy handler, well-written worker, version-aware review) with and without skill
• With-skill scored 18/18 (100%) vs without-skill 15/18 (83%) on assertions
• Key skill wins: slog awareness (baseline never suggests it), false positive resistance (2 vs 7 issues on clean code), consistent output format
• Skill costs ~69% more tokens due to reference loading but quality improvement justifies it

🤖 Generated with Claude Code