NixOS doesn't populate /bin (no /bin/sleep), so the gateway-init
end-to-end tests that spawn a real `sleep` errored with
FileNotFoundError. Add tests/_bin.py with a PATH-resolved SLEEP
constant (falling back to /bin/sleep on FHS hosts) and import it in
test_gateway_init.py instead of hardcoding the path.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A9qa3xoavjQScufDfZaXKR
