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_deepseek.nu

@@ -39,6 +39,11 @@ export def "pj deepseek printkey" [] {
 
 # Query DeepSeek account balance information.
 export def "pj deepseek" [] {
+    if (which ^curl | is-empty) {
+        print "curl is required for `pj deepseek`."
+        return
+    }
+
     if not ($key_file | path exists) {
         print $"(char --unicode '274c') No DeepSeek API key found. Use `pj deepseek setkey <api-key>` to set it."
         return