Add Copilot instructions for repository

.github/copilot-instructions.md

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+# Copilot Instructions for mdview
+
+## Project Overview
+
+mdview is a command-line tool written in Go that formats Markdown files and launches them in a web browser. It converts Markdown to HTML with GitHub-style CSS and supports various features including image embedding, custom output paths, and bare HTML output.
+
+## Technology Stack
+
+- **Language**: Go 1.21.1
+- **Main Dependencies**:
+  - `github.com/pkg/browser` - for opening files in the default browser
+  - `gitlab.com/golang-commonmark/markdown` - for markdown parsing and rendering
+- **Build Tool**: Make
+- **Documentation**: Pandoc (for man page generation)
+
+## Project Structure
+
+- `main.go` - Single-file application containing all core logic
+- `github-markdown.css` - Embedded GitHub-style CSS for rendered output
+- `template.html` - Embedded HTML template for output
+- `mdview.1.md` - Man page in Markdown format (converted to `mdview.1` via pandoc)
+- `Makefile` - Build automation for multiple platforms
+- `.github/workflows/` - CI/CD workflows for building and releasing
+
+## Build Commands
+
+### Building for Development
+```bash
+# Build for current platform
+go build -o mdview .
+
+# Build for specific platform with version
+VERSION=v1.0.0 make linux        # Linux (amd64, arm64, i386)
+VERSION=v1.0.0 make windows      # Windows (amd64)
+VERSION=v1.0.0 make darwin       # macOS (amd64, arm64)
+VERSION=v1.0.0 make freebsd      # FreeBSD (amd64)
+VERSION=v1.0.0 make all          # All platforms
+```
+
+### Building Packages
+```bash
+VERSION=v1.0.0 make deb          # Debian packages (amd64, arm64)
+make snap                         # Snap package
+```
+
+### Testing
+Currently, there is no formal test suite in this project.
+
+### Linting
+```bash
+go vet ./...
+go fmt ./...
+```
+
+## Code Style and Conventions
+
+### General Go Conventions
+- Follow standard Go formatting (use `go fmt`)
+- Use standard Go idioms and practices
+- Keep code simple and readable
+- Use descriptive variable names
+
+### Project-Specific Conventions
+
+1. **Version Handling**:
+   - Version is injected at build time via `-ldflags "-X main.appVersion=$(VERSION)"`
+   - VERSION variable should not include 'v' prefix in Makefile operations
+   - Strip 'v' prefix with `${VERSION#v}` when needed
+
+2. **Error Handling**:
+   - Use the `check()` function for error handling
+   - Provide user-friendly error messages for common errors (permission, file not found)
+   - Include special handling for Snap installations with helpful messages
+
+3. **Embedded Resources**:
+   - CSS and HTML template are embedded using `//go:embed` directives
+   - Keep embedded resources in the root directory
+
+4. **Flag Handling**:
+   - Support both short (`-v`, `-h`, `-b`) and long (`-version`, `-help`, `-bare`) flag forms
+   - Use the `flag` package for command-line argument parsing
+
+5. **File Handling**:
+   - Support custom temp directory via `MDVIEW_DIR` environment variable
+   - Handle Snap installations specially (restricted to HOME directory)
+   - Use proper file permissions (0700 for directories)
+
+6. **Image Processing**:
+   - Convert relative image paths to data URIs for portability
+   - Process both Markdown image syntax and HTML `<img>` tags
+   - Use regex for HTML image processing
+
+## Important Implementation Details
+
+1. **Browser Integration**:
+   - Silence browser stdout/stderr to avoid cluttering output
+   - Use `browser.OpenFile()` after writing HTML
+
+2. **Temporary Files**:
+   - Generate random hex-based filenames for temp files
+   - Respect `MDVIEW_DIR` environment variable
+   - Handle Snap sandboxing restrictions
+
+3. **Markdown Processing**:
+   - Enable HTML, tables, and typographer features
+   - Extract first heading as page title
+   - Process tokens to handle embedded images
+
+## Platform Support
+
+mdview is built for multiple platforms:
+- Linux: amd64, arm64, i386
+- Windows: amd64
+- macOS (Darwin): amd64, arm64
+- FreeBSD: amd64
+
+## Packaging
+
+The project supports multiple package formats:
+- **Debian/Ubuntu**: `.deb` packages via `dpkg-deb`
+- **Snap**: Snapcraft packages (note: sandboxing restrictions apply)
+- **Tarballs**: `.tar.gz` archives for Linux, macOS, FreeBSD
+- **ZIP**: `.zip` archives for Windows
+
+## Release Process
+
+Releases are automated via GitHub Actions (`.github/workflows/release.yml`):
+1. Triggered on tag push
+2. Builds for all platforms
+3. Creates Debian packages
+4. Generates release artifacts
+5. Uploads to GitHub Releases
+
+## Dependencies
+
+When adding new dependencies:
+- Keep dependencies minimal
+- Prefer standard library when possible
+- Update `go.mod` and `go.sum` properly
+- Test cross-platform builds after adding dependencies
+
+## Documentation
+
+- Keep README.md up to date with usage examples
+- Update man page (`mdview.1.md`) when changing command-line flags
+- Man page is generated from Markdown using pandoc
+
+## Common Tasks
+
+### Adding a New Flag
+1. Add flag variable using `flag.String()`, `flag.Bool()`, etc.
+2. Add both short and long forms using `flag.BoolVar()` or similar
+3. Update help text in error message
+4. Update man page (`mdview.1.md`)
+5. Update README.md examples
+
+### Modifying HTML Output
+1. Edit `template.html` for structure changes
+2. Edit `github-markdown.css` for style changes
+3. Both files are embedded at build time
+
+### Adding Platform Support
+1. Add new target in Makefile
+2. Follow existing pattern for `bin/<os>-<arch>/mdview`
+3. Create appropriate archive (tar.gz for Unix-like, zip for Windows)
+4. Update README.md installation section
+
+## CI/CD
+
+- **Build workflow** (`.github/workflows/build.yml`): Runs on all pushes
+- **Release workflow** (`.github/workflows/release.yml`): Runs on tag pushes
+- Both workflows use `make` targets for building
+
+## Things to Avoid
+
+- Don't add test files or test dependencies unless implementing a proper test suite
+- Don't modify the build process without testing all platforms
+- Don't change embedded resources without considering existing users
+- Don't break backward compatibility with command-line flags
+- Don't add unnecessary dependencies
+- Avoid making changes that would affect cross-platform builds