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/delete.go

@@ -72,6 +72,7 @@ func deleteTasks(args []string) error {
 
 	for _, task := range tasks {
 		task.Delete(false) // Soft delete
+		engine.RecordUndo("delete", task.UUID)
 	}
 
 	if len(tasks) == 1 {