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
+}

internal/platform/writefile_test.go

@@ -0,0 +1,54 @@
+package platform
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestWriteUserFileCreatesParent(t *testing.T) {
+	dir := t.TempDir()
+	dest := filepath.Join(dir, "a", "b", "c.txt")
+	if err := WriteUserFile(dest, []byte("hi"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	got, _ := os.ReadFile(dest)
+	if string(got) != "hi" {
+		t.Fatalf("content %q", got)
+	}
+	info, _ := os.Stat(dest)
+	if info.Mode().Perm() != 0o644 {
+		t.Fatalf("mode %v", info.Mode().Perm())
+	}
+}
+
+func TestWriteUserFileOverwritesAtomically(t *testing.T) {
+	dir := t.TempDir()
+	dest := filepath.Join(dir, "x")
+	if err := WriteUserFile(dest, []byte("one"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := WriteUserFile(dest, []byte("two"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	got, _ := os.ReadFile(dest)
+	if string(got) != "two" {
+		t.Fatalf("got %q want two", got)
+	}
+	// No .tmp sibling should be left behind.
+	if _, err := os.Stat(dest + ".tmp"); !os.IsNotExist(err) {
+		t.Fatalf(".tmp left behind: %v", err)
+	}
+}
+
+func TestWriteUserFileExecMode(t *testing.T) {
+	dir := t.TempDir()
+	dest := filepath.Join(dir, "bin")
+	if err := WriteUserFile(dest, []byte("#!/bin/sh\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	info, _ := os.Stat(dest)
+	if info.Mode().Perm() != 0o755 {
+		t.Fatalf("mode %v want 0755", info.Mode().Perm())
+	}
+}