panto-warming

Periodically sends a minimal transient prompt on top of the session's
current context so provider-side prompt caches stay warm between turns,
then discards the prompt and the response -- nothing enters the context
window or the session transcript. It requires a Pantograph host exposing
callback-form agent:run_async(request, callback); older no-callback hosts
are unsupported. Configured through a [warming] section in config.toml,
mirroring opencode's warming feature.