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_zoxide.nu

@@ -42,7 +42,7 @@ export-env {
 #
 
 def _pj_normalize_windows_drive [path: string] {
-  if (($nu.os-info.name | str lowercase) != "windows") {
+  if (($nu.os-info.name? | default "" | str lowercase) != "windows") {
     return $path
   }