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/modules/pj_prompt.nu

@@ -572,7 +572,7 @@ def _pj_prompt_static_base [] {
 
 def _pj_left_prompt [] {
     let p = ($env.PJ_PROMPT_PALETTE? | default (_pj_palette_default))
-    let static = ($env.PJ_PROMPT_STATIC? | default (_pj_prompt_static_base))
+    let static = ($env.PJ_LEFT_PROMPT_STATIC_PART? | default (_pj_prompt_static_base))
 
     let user_color = if ($static.effective_user == "root") { $p.c_root } else { $p.c_user }
 
@@ -635,7 +635,7 @@ export-env {
     load-env {
         PJ_PROMPT_PALETTE: (_pj_palette_default)
         PJ_OS_KEY: (_pj_detect_os_key)
-        PJ_PROMPT_STATIC: (_pj_prompt_static_base)
+        PJ_LEFT_PROMPT_STATIC_PART: (_pj_prompt_static_base)
         PROMPT_MULTILINE_INDICATOR: ":"
         PROMPT_INDICATOR: {||
             _pj_prompt_indicator