feat(extraction): streaming chat turn service and NDJSON endpoint

[aredenba-rh]

Summary

• Add ExtractionChatTurnService with sticky runtime lease, JobPackage readiness gate, and UI-mode skill overlays
• Expose POST /extraction/knowledge-graphs/{kgId}/sessions/{mode}/chat as NDJSON (thinking, wait, done)
• Ship tracer-bullet DeterministicExtractionChatAgent until Claude SDK sticky containers land (feat(extraction): Claude Agent SDK sticky container runtime #742)

Issues

Closes #739
Closes #740

Test plan

• cd src/api && uv run pytest tests/unit/extraction/ -v
• Manual: POST chat turn with prepared/unprepared data sources in Extraction Jobs mode

Made with Cursor

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