feat(nushell): add auto-bootstrapped completion registry flow

- generate cache registry module from command list and load it at startup

- add pj completion sync (-s) with optional help and restart behavior

- keep missing-cache fallback via noop module and preserve cache ignore rules

nushell/cache/.gitignore

@@ -1,2 +1,3 @@
 *
-!.gitignore
+!.gitignore
+!noop.nu

nushell/cache/noop.nu

@@ -0,0 +1 @@
+# Fallback completion module intentionally exports nothing.