fix: protect task_uuid from DispatchContext overwrite and update acceptance criteria

- Skip task_uuid key when merging DispatchContext into dispatch metadata
- Update requirements.md acceptance criteria and test matrix for external rules

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/ai_client/client.go

@@ -204,7 +204,9 @@ func buildDispatchMetadata(task *database.Task) map[string]string {
 		var ctx map[string]string
 		if err := json.Unmarshal([]byte(task.DispatchContext), &ctx); err == nil {
 			for k, v := range ctx {
-				metadata[k] = v
+				if k != "task_uuid" {
+					metadata[k] = v
+				}
 			}
 		}
 	}