feat: add WorkflowResult

[HairlessVillager]

注意：这个 PR 修改了 Eko.execute 的签名，请下游调用者注意适配。

这个 PR 增加了 Workflow Result 的功能，让工作流结束时返回一个结果，包括以下字段：

1. isSuccessful：指示工作流最终是否执行成功；
2. summary：对工作流本身的简短的总结；
3. payload：工作流的产物，目前支持纯文本产物，同时预留了其他格式产物的类型，当工作流需要返回结果时（例如“给我生成一个述职报告”）非空，当工作流不需要返回结果时（例如“帮我打开 OpenAI 的官网”）为空，

以下是实现思路：

1. 新增WorkflowResult类型，涵盖以上字段；
2. 新增DocumentAgentToolInput工具，让它实现文书 Agent 工作，负责生成纯文本产物，通过修改工具的 description 来提示模型在合适的时候调用；
3. 修改SummaryWorkflow工具，新增输入参数isSuccessful，并微调 prompt；
4. 新增关于context.variables的写入逻辑：DocumentAgentToolInput工具会把返回值写入workflow_transcript键中，SummaryWorkflow工具会把返回值写入workflow_is_successful和workflow_summary中；
5. 新增关于context.variables的读取逻辑：WorkflowImpl.execute方法会读取workflow_is_successful, workflow_summary, workflow_transcript的值并组装成WorkflowResult类型的值。

以下是主要的修改：

1. Eko.execute方法和WorkflowImpl.execute方法的返回类型修改为Promise<WorkflowResult>，不再返回Promise<NodeOutput[]>；
2. 修改了生成工作流时的 system prompt（见createWorkflowPrompts()），使其不再工作流最后加入一个 Workflow Summary 的节点，而且不允许使用summary_workflow工具；
3. 在WorkflowGenerator.doGenerateWorkflow方法中，把一个 Workflow Summary 的节点通过硬编码的形式加入工作流；
4. 新增接口/类型：WorkflowResult, WorkflowTranscript, WorkflowArtifact, DocumentAgentToolInput, DocumentAgentToolOutput；
5. 修改接口：SummaryWorkflowInput新增了一个isSuccessful: boolean的字段；
6. 新增DocumentAgentTool工具，用来生成工作流产物（纯文本），通过详尽的参数利用大模型生成长文本；

你可以通过以下方式来验证这个 PR：

1. 构造一个需要输出长文本的任务 prompt（例如Search Sam Altman's information on wikipedia and summarize it, you should only search 1 result）并运行，期望行为是工作流返回值为 WorkflowResult 类型，且isSuccessful为true，summary为对工作流状态的简短总结，payload 为对 Sam Altman 的总结；
2. 构造一个不需要输出、只有操作的任务 prompt（例如Open the official website of OpenAI for me）并运行，期望行为是工作流返回值为 WorkflowResult 类型，且isSuccessful为true，summary为对工作流状态的简短总结，payload 为空。

---

NOTE: This PR modifies the signature of Eko.execute. Please pay attention to compatibility for downstream callers.

This PR enhances the functionality of Workflow Result, enabling it to return a result at the end of a workflow. The result includes the following fields:

1. isSuccessful: Indicates whether the workflow executed successfully in the end;
2. summary: A brief summary of the workflow itself;
3. payload: The output of the workflow. Currently, it supports plain text output and reserves types for outputs in other formats. The payload is non-empty when the workflow needs to return a result (e.g., "Generate a performance report for me") and empty when the workflow does not need to return a result (e.g., "Open the official website of OpenAI for me").

Here is the implementation plan:

1. Add a new WorkflowResult type, encompassing the above fields;
2. Introduce the DocumentAgentToolInput tool to handle document-related tasks and generate plain text outputs. The tool's description will be modified to prompt the model to invoke it at appropriate times;
3. Modify the SummaryWorkflow tool by adding an input parameter isSuccessful and slightly adjusting the prompt;
4. Add logic for writing to context.variables: The DocumentAgentToolInput tool writes the return value to the workflow_transcript key, while the SummaryWorkflow tool writes the return values to workflow_is_successful and workflow_summary;
5. Add logic for reading from context.variables: The WorkflowImpl.execute method reads the values of workflow_is_successful, workflow_summary, and workflow_transcript and assembles them into a WorkflowResult type value.

Here are the main changes:

1. The return types of the Eko.execute and WorkflowImpl.execute methods have been changed to Promise<WorkflowResult>, instead of Promise<NodeOutput[]>;
2. The system prompt when generating a workflow (see createWorkflowPrompts()) has been modified to no longer add a Workflow Summary node at the end of the workflow and to disallow the use of the summary_workflow tool;
3. In the WorkflowGenerator.doGenerateWorkflow method, a Workflow Summary node is hard-coded into the workflow;
4. New interfaces/types added: WorkflowResult, WorkflowTranscript, WorkflowArtifact, DocumentAgentToolInput, DocumentAgentToolOutput;
5. Interface modification: SummaryWorkflowInput has a new field isSuccessful: boolean;
6. Introduced the DocumentAgentTool to generate workflow outputs (plain text) by leveraging a large model with detailed parameters to generate long texts.

You can verify this PR by:

1. Creating a task prompt that requires outputting long text (e.g., "Search Sam Altman's information on Wikipedia and summarize it, you should only search 1 result") and running it. The expected behavior is that the workflow returns a WorkflowResult type value with isSuccessful as true, summary as a brief overview of the workflow status, and payload as a summary of Sam Altman;
2. Creating a task prompt that requires no output and only actions (e.g., "Open the official website of OpenAI for me") and running it. The expected behavior is that the workflow returns a WorkflowResult type value with isSuccessful as true, summary as a brief overview of the workflow status, and payload as empty.

[HairlessVillager]

经过下游调用者审查通过，这个 PR 合并。

---

After being reviewed and approved by downstream callers, this PR will be merged.