refactor: migrate from master to main branch + implement PR workflow

- Rename master branch to main for inclusive language
- Update all documentation references from master → main
- Implement PR-based workflow with maintainer approval required
- Document tea CLI usage for Gitea pull requests
- Establish clear branch hierarchy: feature → integration → main

Branch Strategy:
- main: Production packages (requires PR approval)
- integration: Staging area for multiple packages
- feature/package-[name]: Individual package development

Workflow Pattern:
1. Create feature/package-[name] from integration
2. Develop package in feature branch
3. Merge feature → integration (direct merge)
4. Create PR integration → main (requires approval)

This provides proper quality gates while enabling parallel development
of the 56 applications with maintainer oversight.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

PLAN.md

@@ -75,7 +75,7 @@ Package ~100 applications for KNEL's Cloudron platform to create a comprehensive
 
 ### Git Strategy
 ```
-master (stable packages)
+main (stable packages)
   ↑
 integration (testing multiple packages)
   ↑
@@ -91,7 +91,7 @@ feature/package-[appname] (individual development)
 6. **Document** → Create comprehensive build notes
 7. **Review** → Code review and quality assurance
 8. **Integrate** → Merge to integration branch
-9. **Release** → Promote to master after validation
+9. **Release** → Promote to main after validation
 
 ### Automation Goals
 - [ ] Automated testing of package builds