[idea] Visual divider between space groups in the agents panel (agent_panel_sort = "spaces")

[windrunner-tmc]

What

When the agents panel is sorted by spaces, draw a subtle horizontal rule (or configurable gap) between consecutive workspace groups, so each space's agents read as one visual block. Ideally themable (e.g. drawn in overlay0) and opt-in via something like [ui.sidebar.agents] group_divider = true.

Why

With agents across several workspaces, the panel renders as one flat list: the last agent of one space and the first agent of the next are spaced identically to two agents of the same space, so group boundaries are invisible. Each entry can show its workspace token, but repeating the workspace on every row is noise when what you want is "these three belong together" at a glance — the grouping the spaces sort already implies just isn't visible.

Current workaround (and why it falls short)

Row layouts can fake it: add a [{ token = "$grp" }] row and report a grp metadata token (a string of ─ characters) on one pane per group — rows whose tokens resolve empty are skipped, so only that pane renders the line. But this is fragile in ways config can't fix: the metadata is in-memory and vanishes on server restart, dies with the carrying pane, must be manually re-placed when spaces or agents change, and the divider row inherits the continuation-row indent, so it can't align with entry rows. A first-class boundary marker would eliminate all of that.

Scope notes

Only meaningful for spaces sort; in priority mode entries interleave by attention, so a boundary carries no information there. Prior interest: #1515 proposed the same outcome (closed per the discussions-first process).