Add missing Handlebars join helper for item sheet templates

Class and perk item sheets use {{join arr ", "}} to display array fields
but the helper was never registered.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

module/vagabond.mjs

@@ -367,6 +367,12 @@ Hooks.once("init", () => {
     const n = Number(num) || 0;
     return n >= 0 ? `+${n}` : `${n}`;
   });
+
+  // Join array elements with separator
+  Handlebars.registerHelper("join", (arr, separator) => {
+    if (!Array.isArray(arr)) return "";
+    return arr.join(separator);
+  });
 });
 
 /* -------------------------------------------- */