fix: resolve chat prompt read-only state for subagent sessions

Allows prompting subagent sessions without needing the parent record
Keeps subagents read-only when prompting is disabled
Covers root and subagent cases with unit tests

packages/ui/src/components/chat/chatPromptReadOnly.ts

@@ -0,0 +1,13 @@
+import type { Session } from '@opencode-ai/sdk/v2';
+
+export const resolveChatPromptReadOnly = (
+    session: Session | null | undefined,
+    allowPromptingSubagentSessions: boolean,
+    readOnly: boolean,
+): boolean => {
+    if (session?.parentID) {
+        return !allowPromptingSubagentSessions;
+    }
+
+    return readOnly;
+};