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

src/config/types.base.ts

@@ -144,4 +144,6 @@ export type IdentityConfig = {
   name?: string;
   theme?: string;
   emoji?: string;
+  /** Path to a custom avatar image (relative to workspace or absolute). */
+  avatar?: string;
 };