Add Go CLI with container expansion parser and development server

- Implement Go-based CLI for build-time HTML enhancement
- Add container expansion: div.insertr auto-expands to viable children
- Create servedev command with live development workflow
- Add Air configuration for automatic rebuilds and serving
- Enable transition from runtime JS to build-time enhancement approach

insertr-cli/main.go

@@ -0,0 +1,9 @@
+package main
+
+import (
+	"github.com/insertr/cli/cmd"
+)
+
+func main() {
+	cmd.Execute()
+}