dsh-convo-cost: Real-Time Tracking of Conversation Token Usage and Cost

The `dsh-convo-cost` plugin is used during DeepSeek Harness development and debugging to monitor a session's token consumption and estimated cost in real time. By subscribing to the event stream, it displays the cost and token count in the status line below the input box, with a hover detail showing a detailed breakdown. It calculates costs based on DeepSeek's official pricing and supports automatic switching between base and peak/off-peak rates. After installation, restart `dsh web`; if ¥0.00 is displayed, it is usually due to an unknown model or no events being generated. The displayed amount is an estimate and does not include discounts or platform markups.