feat(gui): fold todo/write into ConversationSnapshot.todos

Session consumes the todo/write session event as a per-event side effect
(last write wins), rebuilds it on window replay/paging/resync, and exposes
snapshot.todos. TodoItem re-exported through the runtime surface.

packages/client/ui-conversation/tests/gate-branch-tails.spec.tsx

@@ -25,7 +25,7 @@ function snapshotBase(): ConversationSnapshot {
   return {
     sessionId: SID, nodes: [], foldDegraded: false, partial: null, runningCalls: [],
     pending: [], running: false, removed: false, openState: 'open', openError: null,
-    hasMore: false, loadingOlder: false, promptError: null, lastAgentError: null,
+    hasMore: false, loadingOlder: false, promptError: null, lastAgentError: null, todos: [],
   } as ConversationSnapshot
 }
 

packages/client/ui-conversation/tests/skeleton-branches.spec.tsx

@@ -29,7 +29,7 @@ function snapshotBase(): ConversationSnapshot {
   return {
     sessionId: SID, nodes: [], foldDegraded: false, partial: null, runningCalls: [],
     pending: [], running: false, removed: false, openState: 'open', openError: null,
-    hasMore: false, loadingOlder: false, promptError: null, lastAgentError: null,
+    hasMore: false, loadingOlder: false, promptError: null, lastAgentError: null, todos: [],
   } as ConversationSnapshot
 }