feat(04-01): generate jobs and admin controllers, reject model imports

Scaffold pact.Job and pact.AdminController stubs with Winter YAML
assets, and fail summer build when models imports a sibling package.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/build/build.go

@@ -37,6 +37,9 @@ func App(ctx context.Context, appDir string, out io.Writer) error {
 	if err := generate(appDir, m); err != nil {
 		return err
 	}
+	if err := checkModelsLeaf(appDir, m); err != nil {
+		return err
+	}
 
 	binRel := filepath.Join("bin", m.Binary)
 	if err := os.MkdirAll(filepath.Join(appDir, "bin"), 0o755); err != nil {