Wire the relay adapter into gateway startup and make activation config-driven
instead of a dark-launch flag.
- gateway/relay/__init__.py: replace relay_enabled()/HERMES_GATEWAY_RELAY with
relay_url() (GATEWAY_RELAY_URL env or gateway.relay_url in config.yaml) — the
same shape as gateway.proxy_url. register_relay_adapter() registers when a URL
is configured and builds a live WebSocketRelayTransport; with no URL it's a
no-op (direct/single-tenant deployments unaffected). force=True keeps the
transport-less adapter for unit tests. relay_platform_identity() reads the
hello platform/botId from GATEWAY_RELAY_PLATFORM/GATEWAY_RELAY_BOT_ID.
- gateway/run.py: call register_relay_adapter() during GatewayRunner.start(),
right after plugin discovery, so a configured connector relay is registered
on every boot. Failures are logged, never block startup.
This removes the dark-launch posture: the relay is on whenever it's configured,
shipping the production end state rather than hiding it behind a flag.
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