feat: new and simplified CLI flag parsing code and YAML support

Author: stainless-app[bot]

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/itchyny/json2yaml v0.1.4
 	github.com/kernel/hypeman-go v0.9.1
 	github.com/muesli/reflow v0.3.0
+	github.com/stretchr/testify v1.10.0
 	github.com/tidwall/gjson v1.18.0
 	github.com/tidwall/pretty v1.2.1
 	github.com/urfave/cli-docs/v3 v3.0.0-alpha6
@@ -73,4 +74,7 @@ require (
 	google.golang.org/genproto/googleapis/api v0.0.0-20250825161204-c5933d9347a5 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20250825161204-c5933d9347a5 // indirect
 	google.golang.org/grpc v1.75.1 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 )