chore: remove Gitea configuration and references

Removed all Gitea-related files and references:
- Deleted .gitea/workflows directory (ai-review, ci, e2e, release)
- Removed gitea MCP server from .mcp.json
- Updated CLAUDE.md to list GitHub instead of Gitea
- Updated CHANGELOG.md to remove Gitea migration note

All CI/CD now runs exclusively through GitHub Actions.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

CHANGELOG.md

@@ -31,7 +31,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - App bar badge, settings buttons, and UI elements now use theme-aware CSS variables
 
 ### Infrastructure
-- Migrated from Gitea to GitHub Actions exclusively
 - Added CI workflow for lint, type-check, build, and test
 - Enhanced E2E testing documentation with comprehensive guides
 - Added documentation-engineer subagent