Add tests and documentation for label/discoverability features

[thomasnemer]

Add comprehensive testing and documentation for:

• Test enhanced 'syld contribute issue' command with various label configurations
• Test auto-detection logic for beginner-friendly labels
• Test new 'label' contribution type functionality
• Document new contribution type in help/README
• Document configurable labels feature
• Add integration tests for different forge label conventions

Depends on: #177, #178 (implementation tasks)
Parent issue: #103