feat(messages): derive messagePrefix from identity.name

When identity.name is configured, use it for the default messagePrefix
instead of hardcoded '[clawdbot]'. Falls back to 'clawdbot' if not set.

This allows users to customize how their bot identifies itself in messages
by setting identity.name in their config or IDENTITY.md.

src/config/types.ts

@@ -891,7 +891,7 @@ export type AudioConfig = {
 };
 
 export type MessagesConfig = {
-  messagePrefix?: string; // Prefix added to all inbound messages (default: "[clawdbot]" if no allowFrom, else "")
+  messagePrefix?: string; // Prefix added to all inbound messages (default: "[{identity.name}]" or "[clawdbot]" if no allowFrom, else "")
   responsePrefix?: string; // Prefix auto-added to all outbound replies (e.g., "🦞")
   groupChat?: GroupChatConfig;
   queue?: QueueConfig;