feat: configurable displayName prefix for agent messages in group chats (#255)

Add optional displayName field to agent config. When set, outbound
agent responses are prefixed (e.g. "💜 Signo: Hello!").

Useful in multi-agent group chats where multiple bots share a channel
and users need to tell them apart.

Closes #252

Written by Cameron ◯ Letta Code

"The details are not the details. They make the design." -- Charles Eames

lettabot.example.yaml

@@ -15,6 +15,7 @@ server:
 
 agent:
   name: LettaBot
+  # displayName: "💜 Signo"  # Prefix outbound messages (useful in multi-agent group chats)
   # Note: model is configured on the Letta agent server-side.
   # Select a model during `lettabot onboard` or change it with `lettabot model set <handle>`.