fix(nushell): improve robustness and clarify abstraction boundaries

- pj_completions: merge into $env.config instead of replacing it wholesale
- pj_completions: use parse-time const for cache dir instead of pwd fallback
- pj_fzf / pj_historyhintcomplete: null-safe keybindings access via ? and default
- pj_helix: replace $env.OS (Windows-only) with $nu.os-info.name; extract shared launcher
- pj_prompt: fix off-by-one in git OID substring (0..7 -> 0..<7)
- pj_prompt: use $nu.os-info.name for OS detection instead of sys host hostname
- pj_prompt: cache git availability in static base, skip per-render which call
- pj_prompt: bind LAST_EXIT_CODE once in right-prompt closure
- pj_brew / pj_uv: add --ignore-errors to hide-env calls
- pj_aliases: convert quote/dadjoke to defs with try/catch error handling
- pj_deepseek: guard curl availability before invoking
- pj_zoxide: add ? to $nu.os-info.name access
- pj_starship: fix null-to-nuon producing "null" string; broaden detection
- nu_polyjuice: load pj_starship module

nushell/modules/pj_completions.nu

@@ -96,11 +96,7 @@ export def "pj completion" [
     }
 
     let repo_url = "https://github.com/nushell/nu_scripts.git"
-    let cache_dir = if ('NU_POLYJUICE_PATH' in $env) {
-        ($env.NU_POLYJUICE_PATH | path join 'cache')
-    } else {
-        (pwd | path join 'nushell' 'cache')
-    }
+    let cache_dir = $pj_cache_dir
     let repo_dir = ($cache_dir | path join 'nu_scripts')
     let completion_dir = ($repo_dir | path join 'custom-completions')
 
@@ -169,7 +165,7 @@ export def "pj completion" [
 }
 
 export-env {
-    $env.config = {
+    $env.config = ($env.config? | default {} | merge {
         completions: {
             quick: true
             partial: true
@@ -180,7 +176,7 @@ export-env {
                 max_results: 100
             }
         }
-    }
+    })
 
     if not ($pj_completion_registry_module | path exists) {
         mkdir $pj_cache_dir