fix(tui): clean up SDK event listeners on unmount

[sauerdaniel]

Summary

Fix memory leak in the TUI App component where SDK event listeners (onStdout, onStderr, onMessage, onFinished) are never unsubscribed.

Problem

In packages/tui/src/app.tsx, the useEffect hook sets up event listeners via sdk.on() calls but the cleanup function only calls sdk.stop() without removing the listeners. This causes:

• Event listener accumulation over time
• Memory retention of closed sessions
• Potential duplicate event handling

Solution

• Store unsubscribe functions returned by sdk.on()
• Call all unsubscribe functions in the cleanup return
• Ensures proper cleanup when component unmounts or session changes

Fixes #8258

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