docs: update ROADMAP with completed features and cleaner structure

- Reorganized into Completed / In Progress / Planned sections
- Added all completed AI Agent features
- Added Developer Experience section with keyboard shortcuts
- Added Success Metrics table with current status
- Streamlined future vision with Syncable Platform integration
- Removed outdated/redundant sections

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

Author: 2 people