fix(test): fail loudly on root with cfg escape hatch for fs_errors

Instead of silently skipping, panic with an actionable error message
when running as root. The test can be excluded entirely via
`RUSTFLAGS='--cfg pdu_test_skip_fs_errors'` for environments like
Claude Code Web where root is unavoidable.

https://claude.ai/code/session_01Ki7wbrgQXWs2GXf8h2hybQ

Author: claude

tests/cli_errors.rs

@@ -106,11 +106,15 @@ fn max_depth_0() {
 }
 
 #[cfg(unix)]
+#[cfg(not(pdu_test_skip_fs_errors))]
 #[test]
 fn fs_errors() {
     if unsafe { libc::geteuid() } == 0 {
-        eprintln!("SKIPPED: fs_errors test cannot work as root (permission checks are bypassed)");
-        return;
+        panic!(
+            "{}\n{}",
+            "error: Attempt to run this test on root environment was detected. Such an environment would affect the accuracy of this test.",
+            "hint: Either run this test on non-root environment or set `RUSTFLAGS='--cfg pdu_test_skip_fs_errors'` to skip this error.",
+        );
     }
 
     let workspace = SampleWorkspace::default();