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/go.mod

@@ -0,0 +1,15 @@
+module github.com/insertr/cli
+
+go 1.23.0
+
+toolchain go1.24.6
+
+require (
+	github.com/spf13/cobra v1.8.0
+	golang.org/x/net v0.43.0
+)
+
+require (
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/spf13/pflag v1.0.5 // indirect
+)