feat: initial commit of Git Automation CLI

- Add comprehensive Git workflow automation tools
- Include branch management utilities
- Add commit helpers with conventional commit support
- Implement GitHub integration for PR management
- Add configuration management system
- Include comprehensive test coverage
- Add professional documentation and examples

internal/util/env.go

@@ -0,0 +1,13 @@
+package util
+
+import (
+	"os"
+)
+
+func GetenvOrDefault(key, def string) string {
+	v := os.Getenv(key)
+	if v == "" {
+		return def
+	}
+	return v
+}