subagents need `@` in opencode

[hchaudhary1]

subagents need @ in opencode
https://opencode.ai/docs/agents/#subagents

[taqtiqa-mark]

This upstream issue is relevant anomalyco/opencode#2965

[Cluster444]

At the moment the subagents are not reference with @ because it needs to be optional for the model to choose how many and of which subagents to run. I'm not even sure if the behavior of @ subagent references is stable, but it also seems like a mechanism to force it to be used, which isn't always appropriate.