feat(chat): distill thread titles after first turn (#471)

[huangcheng]

Summary

• After the first successful assistant reply, the gateway sets an interim title from the clipped first user message, then asynchronously distills a short topic via the agent default model (title_distill.py). Manual renames are preserved.
• Dashboard polls for distilled titles for up to ~90s after each turn; minimal nav refreshes its independent session cache on streamEnd.
• Sidebar session titles use hover marquee scrolling when truncated (SessionRowTitle).
• Documented thread title behavior in docs/api.md; CONTRIBUTING checklist now mentions updating docs/ for product/API changes.

Fixes #471

Test plan

• make all (2726 tests passed)
• uv run pytest tests/unit/gateway/test_title_distill.py -q
• Dashboard: send first message in new chat → sidebar shows clipped title, then distilled title within ~60s
• Hover long distilled title → infinite horizontal scroll; mouse leave → ellipsis restored
• Manually rename session → distill must not overwrite custom title
• HITL resume path: test_hitl_resume_records_final_answer_in_thread_history passes