Implement new 'label' contribution type for label adoption suggestions

[thomasnemer]

Create new 'syld contribute label' (or 'syld contribute discoverability') contribution type that:

1. Suggests helping projects improve their discoverability by proposing adoption of standard labels
2. Recommends opening an issue or PR to the target project proposing they adopt GitHub's 'good first issue' label
3. Optionally suggests other discoverability improvements:
   • Adding a CONTRIBUTING.md file
   • Adding topic tags to the repository
   • Adding a FUNDING.yml for sponsorship visibility

Depends on: #176 (research task), #177 (enhance command)
Parent issue: #103