feat(webui): add custom assistant identity support

Adds the ability to customize the assistant's name and avatar in the Web UI.

Configuration options:
- config.ui.assistant.name: Custom name (replaces 'Assistant')
- config.ui.assistant.avatar: Emoji or letter for avatar (replaces 'A')

Also reads from workspace IDENTITY.md as fallback:
- Name: field sets the assistant name
- Emoji: field sets the avatar

Priority: config > IDENTITY.md > defaults

Closes #1383

src/config/zod-schema.ts

@@ -155,6 +155,13 @@ export const ClawdbotSchema = z
     ui: z
       .object({
         seamColor: HexColorSchema.optional(),
+        assistant: z
+          .object({
+            name: z.string().max(50).optional(),
+            avatar: z.string().max(200).optional(),
+          })
+          .strict()
+          .optional(),
       })
       .strict()
       .optional(),