Session transcripts are silently and permanently deleted after 30 days — no warning, no recovery

The default cleanupPeriodDays: 30 permanently deletes local session transcripts at startup with zero notice. I lost months of working context across multiple concurrent large projects and only discovered it when the sessions were already gone. Conversations carry reasoning and decision context that generated artifacts (CLAUDE.md, plans, roadmaps) do not — they are not disposable cache.

At minimum:

1. Warn before deleting
2. Surface the retention setting during onboarding
3. Offer an archive-instead-of-delete option

Silent permanent data loss should never be a default.