DeepSeek Harness Peak and Off-Peak Billing Plugin

This plugin is specifically designed for DeepSeek Harness (DSH) to monitor and calculate token usage and costs in real time under DeepSeek's official peak/off-peak pricing policy. It supports viewing whale ASCII-style bills via the `/billing` command in the TUI status bar or during conversations, and includes built-in agent tools (`show_billing` and `check_pricing_period`). Billing strictly follows official rules, applies only to the `deepseek-v4` series, uses time-based pricing across peak and off-peak periods (50% discount), and supports session inheritance and deduplication. Installation is simple, ready to use out of the box, and requires no additional configuration.