feat(nushell): re-export completions for use-based startup

- switch completion module imports to export use so externs propagate

- extend managed completion command set and required file checks

- rename prompt static cache env var to PJ_LEFT_PROMPT_STATIC_PART

- load completion module via use in nu_polyjuice

nushell/nu_polyjuice.nu

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