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

nushell/nu_polyjuice.nu

@@ -6,7 +6,7 @@ $env.NU_POLYJUICE_PATH = $env.FILE_PWD
 
 # modules
 use modules/pj_aliases.nu *
-use modules/pj_completions.nu *
+source modules/pj_completions.nu
 use modules/pj_helix.nu *
 use modules/pj_fzf.nu *
 use modules/pj_zoxide.nu *