fix(chat): keep the outgoing conversation still until the next one replaces it

Switching sessions moved the conversation on screen before the swap: the
composer and the status chip followed the live selection and re-shaped a
commit ahead of the timeline, so the pinned outgoing chat jumped; and the
reveal effect re-ran for the outgoing session when its waited flag flipped,
hiding it a few frames before the next one mounted. The chat column now
reads one deferred session, and the reveal runs once per opened session.

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

@@ -0,0 +1,18 @@
+import React from 'react';
+
+/**
+ * The session the chat column is showing — the deferred selection the
+ * timeline renders, not the live store value. The composer and everything
+ * stacked with the timeline read it so the column changes as one: a session
+ * click publishes the live selection first, and a composer that followed it
+ * would change height (changed-files row, todos, queued chips) while the
+ * outgoing timeline is still on screen, shoving that timeline before the swap.
+ */
+export type ChatColumnSession = {
+  sessionId: string | null;
+  directory: string | null;
+};
+
+export const ChatColumnSessionContext = React.createContext<ChatColumnSession | null>(null);
+
+export const useChatColumnSession = (): ChatColumnSession | null => React.useContext(ChatColumnSessionContext);