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.
b991834aefJulius Marminge committed on 9/5/2026, 4:54:28 AMparent76430cd
3 files changedLine totals unavailable