- Added `LLMContextSummaryConfig` (summary generation params: `target_context_tokens`, `min_messages_after_summary`, `summarization_prompt`) and `LLMAutoContextSummarizationConfig` (auto-trigger thresholds: `max_context_tokens`, `max_unsummarized_messages`, plus a nested `summary_config`). These replace the monolithic `LLMContextSummarizationConfig`.
