docs(ai-chat): add not-released-yet banner to Sessions upgrade guide

Author: ericallam

docs/ai-chat/upgrade-guide.mdx

@@ -4,6 +4,13 @@ sidebarTitle: "Upgrade Guide"
 description: "Migrating chat.agent code from the prerelease API to the Sessions-as-run-manager release."
 ---
 
+<Note>
+  **Not released yet.** This guide describes an upcoming release of `chat.agent`
+  on top of the new Sessions primitive. The packages and server-side support
+  are still rolling out — we'll remove this banner once the release ships and
+  publish the matching `@trigger.dev/sdk` prerelease tag.
+</Note>
+
 This guide is for customers who tried `chat.agent` during the prerelease period.
 The public surface of `chat.agent({...})`, `useTriggerChatTransport`,
 `AgentChat`, `chat.store`, `chat.defer`, and `chat.history` is largely