dsh-costcast: Introducing the DeepSeek Harness Plugin

`dsh-costcast` is a cost plugin for DeepSeek Harness, designed to address the rapid accumulation of context costs during frequent local use. It provides a persistent display above the input box, showing in real time the context size, the next step, and the estimated cost for the current turn, while using a bar chart to visualize cost trends. The plugin includes a context warning feature: when context usage exceeds 70%, it prompts the user to start a new session, and it can be clicked to expand for a detailed cost breakdown. It does not read session files, makes no network requests, and requires minimal permissions. Users should note that the plugin hardcodes the model name as `deepseek-flash`, and cost data is reset after a restart.