Agents: Hide or alter `Done` checkmark button when session is already marked as done

[bamurtaugh]

• Mark a session as done
• ✅ It moves to Done section in list, as expected
• ❓ If I open the Done section and select a session, I still see the checkmark at the top to mark as done
  • Clicking the checkmark does nothing
  • I'd either expect the checkmark button to not be there, be grayed out, or switch to a Restore arrow