Replace the removed built-in boot-md hook (#17093) with a how-to that
shows users how to wire up the same behavior themselves via the hooks
system. Uses _resolve_gateway_model() + _resolve_runtime_agent_kwargs()
so the example works against custom endpoints and OAuth providers,
not just the aggregator defaults that the old built-in silently assumed.
Co-authored-by: teknium1 <teknium@users.noreply.github.com>
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