feat: add avatar support for agent identity

- Add avatar field to IdentityConfig type
- Add avatar parsing in AgentIdentity from IDENTITY.md
- Add renderAvatar support for image avatars in webchat
- Add CSS styling for image avatars

Users can now configure a custom avatar for the assistant in the webchat
by setting 'identity.avatar' in the agent config or adding 'Avatar: path'
to IDENTITY.md. The avatar can be served from the assets folder.

Closes #TBD

ui/src/styles/chat/grouped.css

@@ -89,6 +89,12 @@
   color: rgba(134, 142, 150, 1);
 }
 
+/* Image avatar support */
+img.chat-avatar {
+  object-fit: cover;
+  object-position: center;
+}
+
 /* Minimal Bubble Design - dynamic width based on content */
 .chat-bubble {
   position: relative;

ui/src/ui/chat/grouped-render.ts

@@ -105,7 +105,7 @@ export function renderMessageGroup(
   `;
 }
 
-function renderAvatar(role: string) {
+function renderAvatar(role: string, avatarUrl?: string) {
   const normalized = normalizeRoleForGrouping(role);
   const initial =
     normalized === "user"
@@ -123,6 +123,12 @@ function renderAvatar(role: string) {
         : normalized === "tool"
           ? "tool"
           : "other";
+  
+  // If avatar URL is provided for assistant, show image
+  if (avatarUrl && normalized === "assistant") {
+    return html`<img class="chat-avatar ${className}" src="${avatarUrl}" alt="Assistant" />`;
+  }
+  
   return html`<div class="chat-avatar ${className}">${initial}</div>`;
 }