FleetView: Archive completed agents into a collapsed group

Summary

In the FleetView agents list, the Completed section grows unbounded. Sometimes a completed agent represents work that might be revisited soon; other times it's genuinely done and just adds noise. There's currently no way to distinguish the two.

Proposed Feature

Add an Archive action to completed agents. Archiving would move the job into a collapsed "Archived" section below the main Completed list — still visible and resumable, but out of the way.

The mental model:

• Completed — recent/in-progress work I might return to soon
• Archived — older completed work I'm keeping around but don't need front-and-center

Expected Behavior

• Right-click (or action menu) on a completed agent shows an "Archive" option
• Archived agents appear in a collapsed "Archived" group below Completed, expandable on demand
• Archived agents remain resumable via claude --resume <session-id> and from the UI
• Optionally: an "Unarchive" action moves them back to Completed

Why This Helps

Right now the choice is: keep everything visible (noisy) or delete it (permanent). An archive tier gives a middle ground for managing completed work across different time horizons.