docs: add local structure notes

cmd/README.md

@@ -0,0 +1,11 @@
+# cmd layout
+
+This repository keeps executable entrypoints under `cmd/`.
+
+- `cmd/server`: main web process (`go run ./cmd/server`)
+
+Why this exists:
+
+- Keeps the repository root focused on project metadata (`README.md`, `go.mod`, `Dockerfile`, etc)
+- Scales cleanly if more binaries are added later (for example: `cmd/migrate`, `cmd/admin`, `cmd/worker`)
+- Matches common Go repository conventions for multi-binary or growing services