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_brew.nu

@@ -15,10 +15,10 @@ export def --env "pj brewthu" [] {
             HOMEBREW_PIP_INDEX_URL: "https://pypi.tuna.tsinghua.edu.cn/simple"
         }
     } else {
-        hide-env HOMEBREW_API_DOMAIN
-        hide-env HOMEBREW_BOTTLE_DOMAIN
-        hide-env HOMEBREW_BREW_GIT_REMOTE
-        hide-env HOMEBREW_CORE_GIT_REMOTE
-        hide-env HOMEBREW_PIP_INDEX_URL
+        hide-env --ignore-errors HOMEBREW_API_DOMAIN
+        hide-env --ignore-errors HOMEBREW_BOTTLE_DOMAIN
+        hide-env --ignore-errors HOMEBREW_BREW_GIT_REMOTE
+        hide-env --ignore-errors HOMEBREW_CORE_GIT_REMOTE
+        hide-env --ignore-errors HOMEBREW_PIP_INDEX_URL
     }
 }