fix: add chat() API and stop misusing summarize()

[summerx96]

Summary

Separates general instruction-following from summarization so non-summary prompts no longer get a default "Summarize …" system instruction.

Why

Structured extraction/ranking prompts were being sent through summarize(), which injects a default summarization system prompt when system_prompt is omitted. That causes the model to summarize instructions instead of executing them and leads to silent parse failures.

Changes

• Add chat() to SDK + HTTP LLM clients and wrapper.
• Switch non-summary call sites to chat() in memorize/retrieve/patch/crud.
• Fix _summarize_segment() to pass segment text + summary system prompt.

Tests

• Not run standalone for this PR; end-to-end run on the stacked branch (feat/gemini-httpx) with Gemini.

Note

If possible, please preserve commit authorship (avoid rewriting author) for contribution attribution.