fix: add dispatch_context to schema docs and test buildDispatchMetadata

- Add dispatch_context column to requirements.md task schema section
- Export buildDispatchMetadata for test coverage
- Add tests: context forwarding, task_uuid overwrite protection, empty context

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

requirements.md

@@ -47,6 +47,7 @@ Bảng `tasks`:
 - `status` TEXT
 - `attempt_openclaw` INTEGER DEFAULT 0
 - `attempt_smtp` INTEGER DEFAULT 0
+- `dispatch_context` TEXT (JSON metadata từ external rules pipeline, truyền qua OpenClaw dispatch)
 - `last_error` TEXT NULL
 - `next_retry_at` DATETIME NULL (chỉ cần nếu retry xử lý theo scheduler/worker)
 - `created_at` DATETIME