TODO Next: undo a turn — /undo /redo + per-turn snapshots

File tools record before-write state via snapshot hook; Session
captures beforeFiles at turn start and afterFiles at turn end (cap
100 via SnapshotStack), and undo/redo restore files + messages
together. Bash not snapshotted (docs + notice).

src/ui/App.tsx

@@ -674,6 +674,28 @@ export function App({
             push({ kind: 'error', text: e instanceof Error ? e.message : String(e) });
           }
           return;
+        case 'undo': {
+          push({ kind: 'user', text: chosen.trim() });
+          setWorking(true);
+          try {
+            push({ kind: 'info', text: await session.undo() });
+          } catch (e) {
+            push({ kind: 'error', text: e instanceof Error ? e.message : String(e) });
+          }
+          setWorking(false);
+          return;
+        }
+        case 'redo': {
+          push({ kind: 'user', text: chosen.trim() });
+          setWorking(true);
+          try {
+            push({ kind: 'info', text: await session.redo() });
+          } catch (e) {
+            push({ kind: 'error', text: e instanceof Error ? e.message : String(e) });
+          }
+          setWorking(false);
+          return;
+        }
         case 'provider':
           push({ kind: 'user', text: chosen.trim() });
           setOnboarding(true);