fix(tui): stabilize sticky prompt tracking

Keep the latest prompt sticky while the viewport is in live assistant output beyond history, and clear stale sticky state at the real bottom using fresh scroll height.

ui-tui/src/__tests__/viewportStore.test.ts

@@ -35,4 +35,20 @@ describe('viewportStore', () => {
     })
     expect(viewportSnapshotKey(snap)).toBe('0:16:5:40:3')
   })
+
+  it('uses fresh scroll height to clear stale non-bottom state', () => {
+    const handle = {
+      getFreshScrollHeight: () => 20,
+      getPendingDelta: () => 0,
+      getScrollHeight: () => 40,
+      getScrollTop: () => 15,
+      getViewportHeight: () => 5,
+      isSticky: () => false
+    }
+
+    const snap = getViewportSnapshot(handle as any)
+
+    expect(snap.atBottom).toBe(true)
+    expect(snap.scrollHeight).toBe(20)
+  })
 })