[Docs] Update README.md - Phase 4 MicroVM is complete

[marctjones]

Summary

README.md shows Phase 4 MicroVM as incomplete but it's fully implemented.

Changes Needed

• Mark Phase 4 as complete in status section
• Add MicroVM usage examples
• Document ebpf-assist vm subcommands
• Add kernel 5.10 requirement note for MicroVM
• Update architecture diagram to show MicroVM components

Current Status Section

- [x] Phase 3: MCP server for AI assistants
- [ ] Phase 4: MicroVM isolation (optional)  <-- Should be checked