fix: guard .trim() calls on potentially undefined workspaceDir (#24875)

Change workspaceDir param type from string to string | undefined in
resolvePluginSkillDirs and use nullish coalescing before .trim() to
prevent TypeError when workspaceDir is undefined.

src/agents/skills/plugin-skills.ts

@@ -12,10 +12,10 @@ import { loadPluginManifestRegistry } from "../../plugins/manifest-registry.js";
 const log = createSubsystemLogger("skills");
 
 export function resolvePluginSkillDirs(params: {
-  workspaceDir: string;
+  workspaceDir: string | undefined;
   config?: OpenClawConfig;
 }): string[] {
-  const workspaceDir = params.workspaceDir.trim();
+  const workspaceDir = (params.workspaceDir ?? "").trim();
   if (!workspaceDir) {
     return [];
   }