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.

cmd/setup/setup.go

@@ -60,7 +60,7 @@ func install(system bool) error {
 
 	if setupGeteuid() == 0 {
 		fmt.Printf("%s %s\n", ui.Colors.Yellow("⚠"),
-			"Running as root without --system configures only root's home. Use `pmg setup install --system` so all users are covered.")
+			"Running as root without --system does not protect other users. Use `pmg setup install --system` so all users are covered.")
 	}
 
 	if err := config.WriteTemplateConfig(); err != nil {