docs: Clean up DEVELOPMENT_PLAN.md for future versions

- Remove all completed v3.0.0 tasks
- Reorganize for v3.1.0 (WebSocket) and v3.2.0 (Async)
- Add summary of completed features
- Update roadmap with future versions
- Simplify structure for ongoing development

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

Co-Authored-By: Claude <[email protected]>

Author: TexasCoding