Reapply "fix(tui): don't show 'Agent not found' toast for subagents (#6528)"

This reverts commit 97a0fd1d54414e5563d0b47a02666d1d044c2cac.

packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx

@@ -202,7 +202,11 @@ export function Prompt(props: PromptProps) {
 
       syncedSessionID = sessionID
 
-      if (msg.agent) local.agent.set(msg.agent)
+      // Only set agent if it's a primary agent (not a subagent)
+      const isPrimaryAgent = local.agent.list().some((x) => x.name === msg.agent)
+      if (msg.agent && isPrimaryAgent) {
+        local.agent.set(msg.agent)
+      }
       if (msg.model) local.model.set(msg.model)
       if (msg.variant) local.model.variant.set(msg.variant)
     }