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

testdata/samples/openai_key.txt

@@ -0,0 +1,2 @@
+# Test file: synthetic OpenAI key pattern
+OPENAI_API_KEY=sk-proj-ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqr1234