docs: explain legacy thread migration

Document which thread metadata and transcript data migrate, which runtime history does not, and how the fresh provider continuation uses the latest 32,000 characters. Add a safe read-only recovery procedure without claiming an export API.

Audit: D02

Written by GPT-5.6 Sol with Codex.
8b1f745fbbJulius Marminge committed on 9/1/2026, 4:32:53 AMparent4b84a36
3 files changedLine totals unavailable