Adjust bash and sandbox timeout defaults

[F16shen]

Summary

• remove the default execution timeout from bash_exec so shell commands are not cut off after 30 seconds
• add a configurable sandbox pre-execution timeout with a 10 second default
• update security policy templates and tests to cover the new timeout behavior

Testing

• /home/lixin/workspace/aish/.venv/bin/python -m pytest tests/test_bash_executor.py tests/security/policy/test_fallback_risk.py tests/security/sandbox/test_ipc.py -q

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