feat(ui): hydrate session diffs on open

Fetch session-level diffs when a session is opened and keep them updated via session.diff SSE events so UI state stays in sync with server changes.

packages/ui/src/stores/session-events.ts

@@ -7,6 +7,7 @@ import type {
 } from "../types/message"
 import type {
   EventSessionCompacted,
+  EventSessionDiff,
   EventSessionError,
   EventSessionIdle,
   EventSessionUpdated,
@@ -428,6 +429,31 @@ function handleSessionUpdate(instanceId: string, event: EventSessionUpdated): vo
   }
 }
 
+function handleSessionDiff(instanceId: string, event: EventSessionDiff): void {
+  const sessionId = event.properties?.sessionID
+  if (!sessionId) return
+
+  const diffs = event.properties?.diff
+  if (!Array.isArray(diffs)) return
+
+  const existing = sessions().get(instanceId)?.get(sessionId)
+  if (existing) {
+    withSession(instanceId, sessionId, (session) => {
+      session.diff = diffs
+    })
+    return
+  }
+
+  // A diff event can arrive before we have hydrated the session list.
+  // Best-effort: fetch the session record so the diff has somewhere to live.
+  void (async () => {
+    await fetchSessionInfo(instanceId, sessionId, (event as any)?.directory)
+    withSession(instanceId, sessionId, (session) => {
+      session.diff = diffs
+    })
+  })().catch((error) => log.warn("Failed to hydrate session for diff event", { instanceId, sessionId, error }))
+}
+
 function handleSessionIdle(instanceId: string, event: EventSessionIdle): void {
   const sessionId = event.properties?.sessionID
   if (!sessionId) return
@@ -605,6 +631,7 @@ export {
   handleQuestionAsked,
   handleQuestionAnswered,
   handleSessionCompacted,
+  handleSessionDiff,
   handleSessionError,
   handleSessionIdle,
   handleSessionStatus,