feat(01-01): create main.go, test scaffolding, and testdata fixtures

- main.go entry point (7 lines) delegates to cmd.Execute()
- cmd/root.go stub so go build ./... compiles (Plan 05 replaces)
- pkg/providers, pkg/storage, pkg/engine package stubs
- Test stubs with t.Skip() for providers, storage, engine packages
- testdata/samples: openai_key.txt, anthropic_key.txt, multiple_keys.txt, no_keys.txt
- go build ./... and go test ./... -short both exit 0

cmd/root.go

@@ -0,0 +1,8 @@
+package cmd
+
+import "os"
+
+// Execute is a stub. The real command tree is built in Plan 05.
+func Execute() {
+	_ = os.Args
+}