orphaned delegate sessions appear as root sessions after parent deletion

[zatevakhin]

When a root session is deleted, its delegate (child) sessions lose their parent_id and show up in the session picker as root-level entries.

repro:

• create a session that spawns delegates
• delete the root session
• observe orphaned delegates appearing as root sessions in both web UI and TUI picker

Options

1. Soft-delete/archive flag on children — filter out of session picker, keep data in DB (useful for finetuning later)
2. Cascade delete — simple but destroys potentially valuable conversation data
3. Re-parent orphans under a synthetic "Archived" root

Leaning toward (1) — data is worth preserving.