dsh-tool-cassette: Recorder for DeepSeek Harness tool calling

Writing tests for Agent workflows typically involves handling two sources of instability simultaneously: model outputs and external tools. The official `dsh-llm-replay` can already replay the model stream; however, tools such as HTTP, MCP, and databases may still access the network, modify data, or rely on a service that is currently down during testing. Additionally, third-party APIs sometimes return `429` (Too Many Requests) in CI environments. `dsh-tool-cassette` is a community plugin for DeepSeek Harness (hereinafter referred to as DSH). Its approach is straightforward: the first time, the selected tools execute for real, and the normalized results are recorded into a cassette file. Subsequently, the system switches to replay mode, sets real tool invocations to zero, and returns the recorded results to the tool chain based on exact matching.