feat: add annotations, undo system, and schema updates

Add annotations as JSON column on tasks table with Annotate/Denotate
methods and CLI commands. Add undo system backed by change_log with
lightweight undo_stack table (capped at 10 entries). All mutating CLI
commands (add, done, delete, modify, start, stop) now record undo
entries. Undo restores prior task state from change_log data.

Schema changes (in v1 migration):
- annotations TEXT column on tasks
- undo_stack table
- annotations field in change_log triggers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/cmd/add.go

@@ -63,6 +63,8 @@ func addTask(args []string) error {
 		return fmt.Errorf("failed to create task: %w", err)
 	}
 
+	engine.RecordUndo("add", task.UUID)
+
 	displayID, err := engine.AppendTask(task)
 	if err != nil {
 		// Non-fatal: task was created, just can't assign display ID
@@ -113,6 +115,8 @@ func addRecurringTask(description string, mod *engine.Modifier) error {
 		return err
 	}
 
+	engine.RecordUndo("add", instance.UUID)
+
 	displayID, err := engine.AppendTask(instance)
 	if err != nil {
 		fmt.Printf("Created recurring task %s\n", *instance.ParentUUID)