docs: add CLAUDE.md for Claude Code integration (#173)

Add comprehensive CLAUDE.md documentation file to enhance Claude Code integration with development commands, architecture overview, and key patterns for improved productivity.

- Fix formatting consistency using hyphens for all list items
- Correct tool name from mise.toml to mise

Author: koki-develop

CLAUDE.md

@@ -0,0 +1,100 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Development Commands
+
+### Testing
+```bash
+go test ./...                    # Run all tests
+go test ./internal/gat          # Test core functionality
+go test ./internal/prettier     # Test code formatting
+```
+
+### Building
+```bash
+go build                        # Build for current platform
+goreleaser check                 # Validate goreleaser config
+goreleaser release --snapshot --clean  # Build cross-platform binaries
+```
+
+### Linting
+```bash
+golangci-lint run --verbose ./...  # Run linter (configured via mise.toml)
+```
+
+### Running
+```bash
+go run . [file]...               # Run from source
+./gat [file]...                  # Run built binary
+```
+
+## Architecture
+
+### Core Components
+
+**CLI Layer (`cmd/`)**
+- `root.go`: Main cobra command setup with terminal detection logic
+- `flags.go`: All CLI flags and environment variable handling (`GAT_FORMAT`, `GAT_THEME`)
+- `version.go`: Version command implementation
+
+**Core Engine (`internal/gat/`)**
+- `gat.go`: Main `Gat` struct and `Print()` method - handles content detection, lexer selection, and output formatting
+- `formats.go` & `languages.go`: List available output formats and supported languages
+
+**Formatters (`internal/formatters/`)**
+- Wrapper around Chroma formatters for terminal, HTML, JSON, SVG output
+- Includes minified variants for web formats
+
+**Lexers (`internal/lexers/`)**
+- Language detection logic using filename and content analysis
+- Integrates with Chroma lexer registry
+
+**Code Prettification (`internal/prettier/`)**
+- Language-specific formatters (Go, JSON, HTML, CSS, XML, YAML)
+- Registry pattern for extensible formatting support
+- `fallback.go`: Default pass-through for unsupported languages
+
+**Themes (`internal/styles/`)**
+- Custom theme registry including `noop.xml` for no-color output
+- Wraps Chroma's style system
+
+### Key Data Flow
+
+- **Input Processing**: `gat.Print()` reads content and detects MIME type
+- **Content Detection**: Binary vs text, with special handling for images and gzip
+- **Lexer Selection**: Auto-detect language from filename/content or use explicit `--lang`
+- **Content Transformation**: 
+  - Markdown rendering (glamour) if `--render-markdown`
+  - Code prettification if `--pretty`
+- **Output Formatting**: Apply syntax highlighting and format (terminal/HTML/JSON/SVG)
+
+### Special Features
+
+**Image Handling**
+- Sixel encoding for terminal image display
+- Automatic resizing (max 1800px edge, disabled with `--no-resize`)
+- Supports JPEG, PNG, GIF
+
+**Terminal Behavior**
+- Auto-detects piped output and disables colors (unless `--force-color`)
+- Uses `noop` theme for non-terminal output
+- Forces binary output when piped
+
+**Environment Integration**
+- `GAT_FORMAT` and `GAT_THEME` environment variables
+- Terminal detection for appropriate output formatting
+
+## Tool Configuration
+
+The project uses:
+- **mise**: Tool version management (Go 1.24.3, golangci-lint, vhs, goreleaser)
+- **goreleaser**: Cross-platform building and release automation
+- **GitHub Actions**: CI with test/build/lint jobs
+
+## Testing Strategy
+
+Tests focus on:
+- Format and language listing functionality
+- Code prettification for each supported language
+- Error handling and edge cases in prettier modules