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/internal/engine/annotate.go

@@ -0,0 +1,27 @@
+package engine
+
+import "fmt"
+
+// Annotate appends a timestamped annotation to the task and saves.
+func (t *Task) Annotate(text string) error {
+	annotation := Annotation{
+		Timestamp: timeNow().Unix(),
+		Text:      text,
+	}
+	t.Annotations = append(t.Annotations, annotation)
+	return t.Save()
+}
+
+// Denotate removes the most recent annotation from the task and saves.
+// Returns the removed annotation, or an error if there are none.
+func (t *Task) Denotate() (*Annotation, error) {
+	if len(t.Annotations) == 0 {
+		return nil, fmt.Errorf("task has no annotations")
+	}
+	removed := t.Annotations[len(t.Annotations)-1]
+	t.Annotations = t.Annotations[:len(t.Annotations)-1]
+	if err := t.Save(); err != nil {
+		return nil, err
+	}
+	return &removed, nil
+}