feat(nushell): add cached nu_scripts completions bootstrap

- add pj completion command to clone/update nu_scripts cache

- load fixed custom-completions for git/ssh/cargo/uv/rustup from cache

- ignore nushell/cache contents in git while keeping placeholder .gitignore

- source pj_completions at startup and align prompt indicators/newline behavior

.gitignore

@@ -1,2 +1,4 @@
 .zed
 .vscode
+nushell/cache/*
+!nushell/cache/.gitignore