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/display.go

@@ -175,6 +175,18 @@ func FormatTaskDetail(task *Task) string {
 		t.AppendRow(table.Row{"Tags", formatTags(task.Tags)})
 	}
 
+	if len(task.Annotations) > 0 {
+		t.AppendSeparator()
+		for i, ann := range task.Annotations {
+			label := ""
+			if i == 0 {
+				label = "Annotations"
+			}
+			ts := time.Unix(ann.Timestamp, 0).Format("2006-01-02 15:04")
+			t.AppendRow(table.Row{label, fmt.Sprintf("%s  %s", ts, ann.Text)})
+		}
+	}
+
 	return t.Render()
 }