feat(plan): allow skills to be enabled in plan mode (#18817)

Co-authored-by: Jerop Kipruto <jerop@google.com>

packages/core/src/tools/tool-names.ts

@@ -108,6 +108,7 @@ export const PLAN_MODE_TOOLS = [
   LS_TOOL_NAME,
   WEB_SEARCH_TOOL_NAME,
   ASK_USER_TOOL_NAME,
+  ACTIVATE_SKILL_TOOL_NAME,
   EXIT_PLAN_MODE_TOOL_NAME,
 ] as const;