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/command.rs

@@ -386,6 +386,11 @@ pub enum AppCommand {
     Mode { value: ModeValue },
     /// Show or set the messages verbosity.
     Messages { value: Option<MessagesValue> },
+    /// Undo the most recent change, restoring the previous snapshot
+    /// after a confirmation prompt (ADR-0006 Amendment 1).
+    Undo,
+    /// Re-apply the most recently undone change, after confirmation.
+    Redo,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -654,6 +659,8 @@ impl Command {
                 AppCommand::Import { .. } => "import",
                 AppCommand::Mode { .. } => "mode",
                 AppCommand::Messages { .. } => "messages",
+                AppCommand::Undo => "undo",
+                AppCommand::Redo => "redo",
             },
         }
     }