fix: Echo widget useEffect cleanup never calls App.close(), leaving zombie PostMessageTransport listener

[nickytonline]

Background

Echo.tsx manually wires App in a useEffect but the cleanup never calls activeApp.close(), leaving the PostMessageTransport listener alive on unmount. Under React StrictMode's dev-mode double-invoke, the first App instance becomes a zombie listener receiving every host message alongside the live one.

return () => {
  isMounted = false; // only guards callbacks, doesn't close the transport
};

This is the same bug that @modelcontextprotocol/ext-apps 1.7.0 fixed inside the useApp() hook (#631).

Proposed fix

Refactor Echo.tsx to use the useApp() hook from @modelcontextprotocol/ext-apps/react, which handles the connect/close lifecycle correctly and is a better reference implementation for template users. As part of this, update CLAUDE.md / AGENTS.md and skill docs to recommend useApp() as the preferred pattern over manual useEffect wiring.

Alternatively, keep the manual pattern but add defaultApp.close() to the cleanup (only for the internally-owned instance, not the injected app prop used in tests/Storybook).