Add CLAUDE.md for Claude Code development guidance

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## About Leeway
+
+Leeway is a heavily caching meta-build system for Go, Yarn (Node.js), and Docker projects. It accelerates build times through intelligent dependency-aware caching and parallel execution.
+
+## Development Commands
+
+### Building
+```bash
+# Build the leeway binary
+go build .
+
+# Build with version info (for releases)
+go build -ldflags "-X github.com/gitpod-io/leeway/pkg/leeway.Version=dev-$(git rev-parse --short HEAD)" .
+
+# Build using leeway itself (self-hosted build)
+./leeway build //:app
+```
+
+### Testing
+```bash
+# Run all tests
+go test ./...
+
+# Run tests with verbose output
+go test -v ./...
+
+# Run specific test package
+go test ./pkg/leeway/...
+
+# Run specific test
+go test -run TestBuildDockerDeps ./pkg/leeway/
+
+# Run tests with build tags for integration tests
+go test -tags integration ./...
+```
+
+### Linting
+```bash
+# Run golangci-lint (if configured)
+golangci-lint run
+
+# Format code
+go fmt ./...
+
+# Vet code
+go vet ./...
+```
+
+## Architecture Overview
+
+### Core Concepts Hierarchy
+1. **Workspace** (`WORKSPACE.yaml`) - Root configuration defining build environment
+2. **Components** (`BUILD.yaml`) - Individual software modules within the workspace  
+3. **Packages** - Buildable units within components, referenced as `component:package`
+
+### Key Architectural Patterns
+
+**Build System Core** (`pkg/leeway/build.go`):
+- Main build orchestration with dependency resolution
+- Package-type-specific build strategies (Go, Yarn, Docker, Generic)
+- Parallel execution with `errgroup` coordination
+- Build phases: prep → pull → lint → test → build → package
+
+**Workspace Management** (`pkg/leeway/workspace.go`):
+- Component discovery and loading
+- Package linking and dependency resolution
+- Environment manifest computation
+- Variant support for conditional builds
+
+**Caching Strategy** (`pkg/leeway/cache/`):
+- Content-addressable versioning
+- Local and remote cache implementations (S3, GCS)
+- Package dependency-aware cache invalidation
+
+**Configuration System**:
+- YAML-based declarative configuration
+- Build argument substitution with `${var}` syntax
+- Package variants for conditional source/config inclusion
+- Dynamic package generation via JavaScript (`BUILD.js`)
+
+### Package Types and Build Strategies
+
+**Go Packages**:
+- Module-aware builds with `go.mod` support
+- Workspace mode for multi-module repositories
+- Dependency extraction to `_deps/` directory
+- Test coverage collection and reporting
+
+**Yarn Packages**:
+- Offline mirror support for reproducible builds
+- Library vs Application packaging modes
+- yarn.lock management for dependency resolution
+- TypeScript compilation support
+
+**Docker Packages**:
+- Multi-stage builds with dependency injection
+- Image extraction for non-pushed containers
+- Build argument propagation (`DEP_COMPONENT__PACKAGE`)
+- Metadata and provenance embedding
+
+**Generic Packages**:
+- Arbitrary command execution
+- Dependency layout in build directory
+- Flexible packaging with tar compression
+
+### Testing Architecture
+
+The test suite uses fixture-based integration testing with `pkg/testutil`:
+- `CommandFixtureTest` for end-to-end CLI testing
+- Workspace setup with programmatic package definitions
+- Mock implementations for external dependencies (Docker, etc.)
+- Output validation through substring matching and custom eval functions
+
+### Version Computation
+
+Package versions are computed from:
+- Package definition hash (BUILD.yaml content)
+- Source file content hashes
+- Transitive dependency versions
+- Environment manifest (tool versions)
+- Build process version constants
+
+Arguments passed via `-D` flags do NOT affect versions unless declared in `argdeps`.
+
+## Working with the Codebase
+
+### Adding New Package Types
+1. Define config struct implementing `PackageConfig` interface
+2. Add build method `buildXXX()` in `build.go` following existing patterns
+3. Update package type constants and switch statements
+4. Add tests in `build_test.go` with fixture setup
+
+### Modifying Build Process
+- Increment `buildProcessVersions` constant when changing build logic
+- Update phase execution in `executeBuildPhase()` 
+- Consider cache invalidation implications
+
+### Cache Implementation
+- Implement `cache.LocalCache` or `cache.RemoteCache` interfaces
+- Handle package serialization/deserialization
+- Ensure atomic operations for cache consistency
+
+### CLI Commands
+Add new commands in `cmd/` directory following Cobra patterns. Use `getWorkspace()` helper for workspace loading and `applyBuildOpts()` for consistent build option handling.