Initial: setup evaluation environment

SPEC.md

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+# WYSIWYG Markdown Editor - Specification
+
+## Overview
+
+Build a markdown editor with preview: Go backend + React/TypeScript frontend with Tailwind CSS.
+
+**Out of scope:** authentication, user accounts, collaborative editing, version history.
+
+## Backend (Go)
+
+- Standard HTTP server + Cobra for CLI
+- CRUD REST API for markdown files
+- Store files on disk (flat structure, `.md` files only)
+- Errors return 4xx/5xx + JSON with `error` field
+- Comprehensive logging using logrus
+
+### CLI Flags
+
+- `--data-dir`: Storage path (default: `./data`)
+- `--port`: Server port (default: `8080`)
+- `--host`: Bind address (default: `127.0.0.1`)
+
+## Frontend (React + TypeScript + Tailwind)
+
+- Markdown editor with live GFM preview
+- File management: list, create, open, save, delete
+- Three themes (Dark, Light, System) with switcher
+- Responsive (desktop + mobile)
+
+## Development Environment
+
+`flake.nix` (locked to `nixos-25.11`) provides: go, gopls, golangci-lint, eslint, nodejs, gnumake, lsof
+
+**Do not modify flake.**
+
+## Testing (TDD)
+
+- **All tests must pass**
+- Single command to run tests
+- Backend: demonstrate CRUD round-trip + static asset serving
+- Frontend: verify core functionality
+
+## Milestones
+
+### Backend (6 milestones)
+
+**B1: CLI & Server Setup**
+- Cobra CLI with --data-dir, --port, --host flags
+- HTTP server with basic routing
+
+**B2: CRUD API**
+- REST endpoints for markdown files (GET, POST, PUT, DELETE)
+- JSON error responses (4xx/5xx)
+
+**B3: File Storage**
+- Read/write .md files to disk
+- Flat file structure
+
+**B4: Logging**
+- Comprehensive logrus logging for all operations
+
+**B5: Static Assets**
+- Serve frontend build files
+
+**B6: Backend Tests**
+- CRUD round-trip tests
+- Static asset serving tests
+
+### Frontend (6 milestones)
+
+**F1: Project Setup**
+- React + TypeScript + Tailwind configured
+
+**F2: File Management UI**
+- List, create, open, save, delete markdown files
+
+**F3: Editor & Preview**
+- Markdown editor with live GFM preview
+
+**F4: Theme System**
+- Dark/Light/System themes
+- Theme switcher and persistence
+
+**F5: Responsive Design**
+- Works at 320px and 1920px
+
+**F6: Frontend Tests**
+- Core functionality tests (editor, file operations, themes)
+
+### Integration (1 milestone)
+
+**I1: End-to-end**
+- Full CRUD workflow test from frontend to backend
+
+## Evaluation
+
+1. CLI starts with defaults
+2. CRUD works end-to-end
+3. Static assets are properly served
+4. Theme switch & persistence
+5. Responsive at 320px and 1920px