feat(plan): add core logic and exit_plan_mode tool definition (#18110)

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

@@ -25,6 +25,7 @@ export const ACTIVATE_SKILL_TOOL_NAME = 'activate_skill';
 export const EDIT_TOOL_NAMES = new Set([EDIT_TOOL_NAME, WRITE_FILE_TOOL_NAME]);
 export const ASK_USER_TOOL_NAME = 'ask_user';
 export const ASK_USER_DISPLAY_NAME = 'Ask User';
+export const EXIT_PLAN_MODE_TOOL_NAME = 'exit_plan_mode';
 
 /**
  * Mapping of legacy tool names to their current names.
@@ -92,6 +93,7 @@ export const PLAN_MODE_TOOLS = [
   LS_TOOL_NAME,
   WEB_SEARCH_TOOL_NAME,
   ASK_USER_TOOL_NAME,
+  EXIT_PLAN_MODE_TOOL_NAME,
 ] as const;
 
 /**