fix: harden system dirs at install; keep sudo attribution without passwd

Address remaining review comments:

- shim: force root:root 0755 on the managed system dirs (shim tree and
  profile.d) after MkdirAll. A pre-created dir with weaker ownership,
  possible under Debian's group-writable /usr/local/lib, would let a
  non-root user replace the shims every account executes.

- audit: when SUDO_USER has no passwd entry (minimal containers),
  attribute cloud events from sudo's recorded SUDO_USER/SUDO_UID env
  instead of falling back to root. Still gated on euid 0.

- setup: reword the root-without---system warning; alias/shim install
  follows HOME, so claiming it configures only root's home was wrong.

- shim: skip the non-root-owner validation test on Windows, where file
  ownership is not resolvable.

internal/shim/system.go

@@ -207,6 +207,24 @@ func ValidateSystemBinary(path string) error {
 	return validateSystemExecutable(path)
 }
 
+// secureSystemDir forces root ownership and 0755 on a directory pmg manages
+// system-wide. MkdirAll leaves pre-existing directories untouched, so a dir
+// pre-created with weaker ownership (possible under Debian's group-writable
+// /usr/local/lib) would let a non-root user replace shims; this closes that
+// hole. No-op when not running as root (unit tests, dry contexts).
+func secureSystemDir(path string) error {
+	if os.Geteuid() != 0 {
+		return nil
+	}
+	if err := os.Chown(path, 0, 0); err != nil {
+		return fmt.Errorf("failed to set root ownership on %s: %w", path, err)
+	}
+	if err := os.Chmod(path, 0o755); err != nil {
+		return fmt.Errorf("failed to set permissions on %s: %w", path, err)
+	}
+	return nil
+}
+
 func shimsPresent(dir string) bool {
 	_, ok := firstShimContent(dir)
 	return ok
@@ -246,6 +264,9 @@ func writeSystemProfile(binDir string) error {
 	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
 		return fmt.Errorf("failed to create profile.d directory: %w", err)
 	}
+	if err := secureSystemDir(filepath.Dir(path)); err != nil {
+		return err
+	}
 
 	content := fmt.Sprintf(`# %s - managed by pmg setup install --system
 # remove by running: pmg setup remove --system