Function save_detached_session_draft 

pub fn save_detached_session_draft(
    workspace_id: &str,
    session_id: &str,
    source: &str,
    owner_pid: u32,
    draft: DetachedSessionDraft,
) -> Result<Option<String>>

Preserve unsent input for explicit recovery and retire its unchanged legacy seed together. Empty input still retires the seed: clearing is an edit.