feat: go lint modernize

[franklinkim]

Description

Enable the golangci-lint modernize linter and apply its suggested fixes across the codebase.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📝 Documentation
• ♻️ Refactoring
• ⚡ Performance
• ✅ Tests
• 🔧 Build/CI

Changes

• Enable the modernize linter in .golangci.yaml (removed from the disabled list)
• Replace interface{} with any throughout (content, client, pkg/handler)
• Convert C-style counter loops to for range n / for i := range n
• Replace manual membership loops with slices.Contains (content/reponode.go, pkg/repo/loader.go)
• Use fmt.Appendf/fmt.Append instead of []byte(fmt.Sprintf(...))

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.

Notes

All changes are mechanical, produced by golangci-lint autofixes; no behavioral changes intended.