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/modify.go

@@ -114,6 +114,7 @@ func modifyTasks(filterArgs, modifierArgs []string) error {
 		if err := mod.Apply(task); err != nil {
 			fmt.Fprintf(os.Stderr, "Warning: failed to modify task %s: %v\n", task.UUID, err)
 		} else {
+			engine.RecordUndo("modify", task.UUID)
 			modified++
 		}
 	}