* fix: prevent FD leaks in child process cleanup
- Destroy stdio streams (stdin/stdout/stderr) after process exit
- Remove event listeners to prevent memory leaks
- Clean up child process reference in moveToFinished()
- Also fixes model override handling in agent.ts
Fixes EBADF errors caused by accumulating file descriptors
from sub-agent spawns.
* Fix: allow stdin destroy in process registry cleanup
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Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>
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