Add the avatar vocabulary to UINode (UI capability platform)

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

conformance_test.go

@@ -125,6 +125,12 @@ func TestGoldenCardEvents(t *testing.T) {
 }
 
 func TestGoldenUIVocabularies(t *testing.T) {
+	golden(t, "ui_avatar", []UINode{
+		{Type: UIAvatar, Avatar: &UIAvatarSpec{Name: "weichen", Sub: "OWNER"}},
+		{Type: UIAvatar, Avatar: &UIAvatarSpec{
+			Stack: []UIAvatarSpec{{Name: "甲", Tone: "ok"}, {Name: "乙", Tone: "warn"}, {Name: "丙"}},
+		}},
+	})
 	golden(t, "ui_vocabularies", []UINode{
 		{Type: UIProgress, Progress: &UIProgressSpec{Label: "市场可行性 78%", Value: 0.78, Tone: "ok"}},
 		{Type: UICompare, Compare: &UICompareSpec{