fix: preserve host signal handlers on shutdown

[obckelbley]

Summary

• track the SIGINT and SIGTERM handlers installed by Sidequest
• remove only those handlers during engine cleanup
• preserve signal handlers owned by the host application, including when Sidequest graceful shutdown is disabled

Why

Engine.close() always calls clearGracefulShutdown(). That function currently uses process.removeAllListeners() for SIGINT and SIGTERM, so stopping Sidequest can silently delete unrelated application shutdown handlers. The same happens when gracefulShutdown is false and Sidequest did not install any handlers itself.

Validation

• corepack yarn workspace @sidequest/engine test:ci
• corepack yarn lint
• corepack yarn format:check
• corepack yarn workspace @sidequest/engine build