feat: ADR-0006 §8 steps 4-5 — undo/redo commands + confirm-modal flow

Commands & grammar (step 4):
- AppCommand::Undo/Redo, grammar nodes + REGISTRY entries, catalog
  help/usage + keys; parse tests
- replay skips undo/redo (is_app_lifecycle_entry_word) + completion
  entry-keyword lockstep; replay-skip test extended

Wiring (step 5):
- Action::{PrepareUndo,PrepareRedo,Undo,Redo} + AppEvent::{UndoPrepared,
  UndoUnavailable,UndoSucceeded,UndoFailed}
- App: undo_enabled flag, Modal::UndoConfirm, dispatch + event handling
  + confirm-key handler (Y confirms / N/Esc cancels); "turned off" when
  --no-undo; "nothing to undo/redo" when empty
- ui::render_undo_confirm names the command + snapshot time
- runtime: opens with undo enabled (!--no-undo), threads it through the
  project-switch path, spawn_prepare_undo/spawn_undo (peek->modal,
  restore->refresh tables + schema cache)
- 9 Tier-1 app tests + 3 parse tests

1692 passed / 0 failed / 1 ignored; clippy clean.

src/dsl/walker/mod.rs

@@ -2750,6 +2750,22 @@ mod tests {
         assert_eq!(parse("rebuild").unwrap(), Command::App(AppCommand::Rebuild));
     }
 
+    #[test]
+    fn walker_parses_undo() {
+        assert_eq!(parse("undo").unwrap(), Command::App(AppCommand::Undo));
+    }
+
+    #[test]
+    fn walker_parses_redo() {
+        assert_eq!(parse("redo").unwrap(), Command::App(AppCommand::Redo));
+    }
+
+    #[test]
+    fn walker_parses_undo_case_insensitive() {
+        // Keywords are case-insensitive (ADR-0009).
+        assert_eq!(parse("UNDO").unwrap(), Command::App(AppCommand::Undo));
+    }
+
     #[test]
     fn walker_parses_new() {
         assert_eq!(parse("new").unwrap(), Command::App(AppCommand::New));