Deleting a thread leaves its descendant threads with a dangling parent

[Tryanks]

Summary

Deleting a thread leaves its descendant thread records in the index with a parent_session_id that no longer exists.

Where

• delete_session (crates/runtime/src/app/sessions.rs) removes only the target record and log. close_orchestrator_children (crates/runtime/src/app/lifecycle.rs) stops the children's processes but keeps their records.
• Archive and unarchive already cascade over descendant_session_ids; delete does not.
• The sidebar tolerates the dangling parent, so the orphans show up as roots or linger in the archived list.

Found by reading the code; not reproduced.

Expected

Delete cascades over the same descendant tree as archive, in one command, and the confirmation states how many threads will be deleted.