[idea] A third agent panel sort: group by space, order the groups by attention

[SteveHolloway71]

idea / problem

The agent panel sorts two ways and they trade against each other. spaces keeps agents grouped with the rest of their space, which is how I think about my work. priority puts what needs me at the top, which is how I triage. Today I have to pick one and lose the other.

What I can't ask for is the combination: keep the grouping, but order the groups by attention.

requested change

A third value for ui.agent_panel_sort — call it spaces-by-priority — that groups exactly as spaces does today and orders the groups by the most demanding agent inside each one. Agents stay with their space; the space that needs me floats to the top.

Nothing changes for the two existing values, and this would be opt-in.

why this is a small ask

herdr already computes the input. Every workspace in session.snapshot carries an aggregate agent_status, and the Spaces panel already renders it as a state_icon per space. So the ordering key exists and is already on screen — what is missing is the option to sort by it.

why it matters at ~30 agents

I run 28 agent panes across 11 spaces on one machine. The agents needing attention are typically scattered across three or four different spaces at once, and which ones they are changes minute to minute.

Under spaces I scroll past the spaces that are fine to find them. Under priority I get them immediately, but the panel becomes one flat run of 28 rows and I lose the structure that tells me what a given agent is for. Putting { token = "workspace" } on every row gets the space name back, but not the grouping — adjacency is the thing that carries meaning here, and per-row labels don't replace it.

The combination is strictly better than either for this shape of work: attention decides where you look, grouping decides what you understand when you get there.

related, and why this isn't a duplicate

• Agent panel: nest agents under one workspace header when grouped #3289 (nest agents under one workspace header) is about how a group is drawn — headers and indentation. This is about what order the groups come in. They compose well and neither needs the other.
• [idea] Let plugins add a mode to the agent panel sort control #3248 (let plugins add a mode to the sort control) is the general version, and I'd rather have that than this — if a plugin could register a mode I would build this one myself and stop asking. But it's a much larger change to the plugin surface, and this specific mode is useful on its own if the extensible version is a way off.
• Sidebar: drag-to-reorder agent panel entries (like spaces) #3144 (drag-to-reorder agent rows) is manual ordering. This is automatic and needs no persistence.

what I tried first

Before filing I checked whether this was already buildable from outside. It nearly is: workspace.move_block and workspace.report_metadata are both in the socket API, so a plugin can reorder spaces and annotate their rows. But reordering the actual spaces to change their display order has side effects a sort option wouldn't — it moves the underlying spaces for every other consumer, and a publisher that dies leaves them shuffled, because a move has no TTL the way reported metadata does.

That's the difference worth naming: this wants a display sort, not a data move. Doing it from outside means mutating real state to influence presentation, which is the wrong shape and the reason I'm asking rather than shipping it.