Agents: Rename archiving to done

[lostintangent]

Within a session, we currently use the terminology Mark as Done to indicate completion of the task. When you click this button, we simply archive the session and then open a new session (to help to start the next task).

That said, it's confusing that we have two terms to mean the same thing: archive, and done. We want to be consistent here, and so we'll orient around Mark as Done, and rename the current archiving UX in the following ways:

1. Non-archived sessions

   1. Rename the Archive context menu on session list items, so that it's Mark as Done instead. But it should continue to have the same behavior.
   2. Update the inline archive action (when hovering over a session list item), so that it uses a checkmark icon instead of the archive icon. Also, it's tooltip should be updated to Mark as Done.
   3. Same as above, but for the inline archive action on a project group node (when the list is grouped by projects). It should use the checkmark-all codicon icon, and have a tooltip value of Mark All as Done.

2. Archived sessions

   1. Rename the Archive session group to Done, but keep it sorted to the bottom of the list.
   2. Update the context menu for archived sessions, so that they have a Mark as Not Done item, as opposed to Unarchive. Additionally, update the inline action so it uses the discard icon and has a tooltip of Mark as Not Done.
   3. Update the Unarchive All action on the Archived group node, so that it uses the discard icon and has a Mark All as Not Done tooltip.

[lostintangent]

Fixed!