Why: plugins and the host need one shared avatar primitive for member/
owner displays in cards, messages, and admin surfaces (user decision
2026-09-30: avatars, cards, settings become base capabilities plugins
reuse directly).
Evidence: go test green with regenerated golden (single avatar with
sub/tone plus a stacked group); omitempty keeps existing wire samples
byte-stable.
Impact: additive optional UINode field + type; unknown-type consumers
ignore it (forward compat). Renderers gain 'avatar' via the ui repo.
Refs: Agent-System-Documents proposal 2026-09-30-ui-redesign-and-auth.md
weichen