[idea] Legend / hover-tooltip for the sidebar agent state dots

[atlas-architect]

The grouped-agents sidebar encodes agent state in dot color/shape (solid teal, solid yellow, hollow circle...) but there is no legend or hover tooltip anywhere in the UI, so new users have to reverse-engineer the vocabulary. We mapped it empirically today by correlating dots against herdr agent list statuses (teal = done/idle-unseen, yellow = working, hollow = idle-seen) — a hover tooltip on the dot (or a one-line legend in the sidebar header / settings) would make the sidebar self-describing. Child-space rows whose occupant herdr cannot detect (e.g. an agent inside wsl.exe) could tooltip that too ('no agent detected in this pane') — today that renders as a bare dot that reads like a bug. Version: 0.7.5-preview.2026-07-29 (Windows).