default group and channel sessions to per-user isolation, allow opting back into shared room sessions via config.yaml, and document Discord gateway routing and session behavior.
Follow-up on salvaged PR #975.
Bridge quick_commands from config.yaml into load_gateway_config(),
normalize non-dict quick command config at runtime, and add coverage
for GatewayConfig round-trips plus config.yaml bridging. This makes the
GatewayConfig quick-command fix complete for the real user-facing config
path implicated by issue #973.