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_historyhintcomplete.nu

@@ -26,17 +26,14 @@ const alt_uppercasel_history_completion = {
 }
 
 export-env {
-    let has_alt_l_history_completion = $env.config.keybindings | where name == "alt_l_history_completion" | is-not-empty
-    if not $has_alt_l_history_completion {
-        $env.config.keybindings = $env.config.keybindings | append [
-            $alt_l_history_completion
-        ]
+    mut existing = ($env.config.keybindings? | default [])
+
+    if not ($existing | where name == "alt_l_history_completion" | is-not-empty) {
+        $existing = ($existing | append [$alt_l_history_completion])
+    }
+    if not ($existing | where name == "alt_uppercasel_history_completion" | is-not-empty) {
+        $existing = ($existing | append [$alt_uppercasel_history_completion])
     }
 
-    let has_alt_uppercasel_history_completion = $env.config.keybindings | where name == "alt_uppercasel_history_completion" | is-not-empty
-    if not $has_alt_uppercasel_history_completion {
-        $env.config.keybindings = $env.config.keybindings | append [
-            $alt_uppercasel_history_completion
-        ]
-    }
+    $env.config = ($env.config? | default {} | upsert keybindings $existing)
 }