Model spec used only for context-compaction summary generation.
None or an absent key keeps the graph's startup summary model.
INHERIT_SUMMARIZATION_MODEL explicitly selects the main agent model. This
value never changes the main model or its system-prompt identity: its only
consumers are the summary-generation slots installed by
offload_middleware._summarization_for_runtime, so compaction thresholds
and token counting still track the main model.
