feat(nushell): gate cache clone behind update flag

- only clone missing nu_scripts cache when using -u/-f

- keep default pj completion as non-cloning check mode

- align completion flag help text with new behavior

nushell/modules/pj_completions.nu

@@ -55,8 +55,8 @@ def _pj_completion_registry_lines [] {
 
 def _pj_completion_help [] {
     print "pj completion commands:"
-    print "  pj completion             Clone/update cache and check required completion files."
-    print "  pj completion -u          Update cached nu_scripts repository."
+    print "  pj completion             Check required completion files from existing cache."
+    print "  pj completion -u          Update cache, or clone nu_scripts when cache is missing."
     print "  pj completion -f          Force re-clone cached nu_scripts repository."
     print "  pj completion sync        Regenerate cache registry and restart Nushell."
     print "  pj completion -s          Same as `pj completion sync`."
@@ -80,8 +80,8 @@ export def "pj completion sync" [
 }
 
 export def "pj completion" [
-    --update(-u) # Pull latest changes when repo already exists.
-    --force(-f) # Re-clone even when repo already exists.
+    --update(-u) # Update cached repo, or clone when cache is missing.
+    --force(-f) # Force re-clone cached nu_scripts repository.
     --sync(-s) # Regenerate static registry and restart Nushell.
     --help(-h) # Show help.
 ] {
@@ -120,8 +120,19 @@ export def "pj completion" [
     }
 
     if not ($repo_dir | path exists) {
+        if ($update or $force) {
             print "[pj completion] nu_scripts cache not found, cloning..."
             ^git clone $repo_url $repo_dir
+        } else {
+            print "[pj completion] nu_scripts cache not found. run `pj completion -u` to clone it."
+            return {
+                cache_dir: $cache_dir
+                repo_dir: $repo_dir
+                completion_dir: $completion_dir
+                commands: $pj_completion_commands
+                missing_files: []
+            }
+        }
     } else {
         if not (($repo_dir | path join '.git') | path exists) {
             print "[pj completion] existing cache is not a git repo, re-cloning..."