The handback logic was appended to the hermes_cli/gateway.py facade; it now lives in a
topical sibling. Supervisor detection also reads the gateway's own declaration (control
socket `identify` -> supervisor: "external", then the live argv marker, then the argv the
gateway stamped into gateway_state.json) so a gateway whose command line cannot be read via
psutil is still handed back rather than SIGTERMed and shadowed by a foreground run.
Tests trimmed to the two invariants (handback with fresh-PID success; either failure branch
never takes ownership) plus the plain-manual control. Docs: `hermes gateway restart` is now
part of the --external-supervisor contract.
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