docs: comprehensive Phase 6 M2 session summary

Documents all work completed in this session:
- Built-in command redirections (48e8dae)
- Undo/redo integration (fa2b6c9)
- Test results, architecture decisions, next steps

Status: 5/7 Phase 6 M2 criteria complete
Next: Property tests (2 days), then Phase 0 sealing (1-2 weeks)

Author: hyperpolymath