Make the initial greeting / empty-state / first assistant message configurable

[mdbecker]

Hi, one small feature request that would make the agent templates much easier to customize:

Right now the initial chat greeting / empty-state message appears to be hardcoded in the shared frontend. That makes a very common customization surprisingly difficult for downstream users of templates like agent-openai-agents-sdk.

In my case, I wanted to change the default welcome text to an app-specific intake message. The backend can control the agent response, but not the very first message shown by the chat UI, so this ends up requiring frontend patching/overrides instead of simple configuration.

It would help a lot if the initial greeting were configurable, for example via one of these:

• app config / env var
• a simple frontend config file
• optional backend-provided initial message

Even a very small version of this would be useful:

• configurable empty-state greeting text, or
• configurable first assistant message for a new chat

Why this would help:

• this is a very common customization
• it avoids patching the shared frontend for a simple UX change
• it makes the templates easier to adopt without changing the overall architecture

This feels like a small, high-impact improvement to the current template system.