Air is a command-line live-reload utility for Go application development. It watches a project directory for file changes, automatically rebuilds the project using a configurable build command, and restarts the resulting binary. The tool includes an embedded HTTP reverse proxy that injects a JavaScript snippet into HTML responses, enabling automatic browser reload via Server-Sent Events after each successful rebuild. It supports custom build commands, pre/post hooks, environment file loading, platform-specific build overrides, and watch rules that run commands instead of triggering a full rebuild (e.g., for frontend asset pipelines or code generation). The architecture is a single Go binary with a main event loop that uses filesystem notifications (or polling) to detect changes, spawns build processes, and manages child process lifecycle. The codebase is well-structured with clear separation of concerns: configuration parsing (runner/config.go), engine orchestration (runner/engine.go), file watching (runner/watcher.go), HTTP proxy (runner/proxy.go), and platform-specific process management (runner/util_*.go). It includes a comprehensive test suite and CI pipelines for Linux, macOS, and Windows. The proxy component, in particular, demonstrates sophisticated handling of content encoding (gzip, brotli), streaming responses (SSE, chunked transfer), and hop-by-hop header management, making it a reusable pattern for adding live-reload capabilities to any Go web application.