feat(platform): add WriteUserFile atomic user-space writer

internal/platform/writefile.go

@@ -0,0 +1,25 @@
+package platform
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// WriteUserFile writes data to path atomically (tmp + rename) with the
+// given mode, creating any missing parent directories (mode 0755).
+// Intended for user-owned destinations under $HOME — no sudo.
+func WriteUserFile(path string, data []byte, mode os.FileMode) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return err
+	}
+	tmp := path + ".tmp"
+	if err := os.WriteFile(tmp, data, mode); err != nil {
+		os.Remove(tmp)
+		return err
+	}
+	if err := os.Rename(tmp, path); err != nil {
+		os.Remove(tmp)
+		return err
+	}
+	return nil
+}