docs: Add comprehensive design document for file editor & chat UI redesign
This document outlines the complete plan for implementing:
- CodeMirror 6 file editor with syntax highlighting
- Enhanced chat input with attachments, history, slash commands
- Hybrid session flow (OpenCode + CodeNomad patterns)
- Mobile-first responsive design
- Conduit-copy integrations (session forking, token tracking)
Key features:
- Phase-by-phase implementation plan (5 phases, 3 weeks)
- Component specifications with code examples
- API endpoints required for new features
- Testing strategy with unit/integration tests
- Proof verification checkpoints
- Mobile device testing matrix
The design incorporates best practices from:
- CodeServer (CodeMirror 6 integration)
- CodeNomad (sophisticated prompt input)
- OpenCode (session management)
- Conduit-Copy (multi-agent orchestration)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
