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

testdata/golden/ui_avatar.json

@@ -0,0 +1,27 @@
+[
+  {
+    "type": "avatar",
+    "avatar": {
+      "name": "weichen",
+      "sub": "OWNER"
+    }
+  },
+  {
+    "type": "avatar",
+    "avatar": {
+      "stack": [
+        {
+          "name": "甲",
+          "tone": "ok"
+        },
+        {
+          "name": "乙",
+          "tone": "warn"
+        },
+        {
+          "name": "丙"
+        }
+      ]
+    }
+  }
+]