Agent panel: nest agents under one workspace header when grouped

[lhrst]

With agent_panel_sort = "spaces", agents from the same workspace are adjacent but each entry repeats the workspace name. With many agents this reads as a flat wall.

Proposal: when grouped by spaces, render one workspace header row and indent its agents beneath it (like worktree children in the Spaces panel), e.g.

ops
 ├ ● claude · title
 └ ○ claude · title

Collapsing a header could hide its agents.

(Originally filed as #3288, redirected here per triage.)