docs: update README with current architecture and features (#64)

- Add comprehensive feature overview with current capabilities
- Update architecture section with new components (dashboard, webhooks, tasks)
- Modernize installation and setup instructions
- Add detailed project structure and development workflow
- Include advanced usage sections for webhooks and task system
- Replace outdated developer checklist with current status
- Add performance optimization documentation for cached GitHub client

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Co-authored-by: Claude <[email protected]>

Author: snomiao