fix: keep Nushell startup resilient and correct OS icon

- Gate cached nu_scripts completion modules on per-version validity
  markers so an outdated completion file no longer aborts the whole
  config load; incompatible files automatically fall back to noop.
- Add 'pj completion doctor' to report enabled/disabled completions.
- Detect macOS via 'macos'/'mac'/'darwin' so the prompt shows the Apple
  icon instead of the Unknown symbol.

nushell/modules/pj_prompt.nu

@@ -1,6 +1,6 @@
 def _pj_detect_os_key [] {
     let os_name = ($nu.os-info.name? | default "" | into string | str lowercase)
-    if ($os_name | str contains "darwin") {
+    if (($os_name | str contains "macos") or ($os_name | str contains "darwin") or ($os_name | str contains "mac")) {
         return "Macos"
     }
     if ($os_name | str contains "windows") {