Add ulimit-based resource caps inside the bwrap sandbox to prevent
fork bombs and resource exhaustion:
- max 64 processes (stops fork bombs)
- 512 MB virtual memory
- 50 MB max file size
- 256 open file descriptors
Limits are applied via `sh -c 'ulimit ...; exec "$@"'` wrapper inside
the sandbox, so they're inherited by all child processes.
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