dsh-switch-cost: Agent Conversation Cost Comparison Plugin

The `dsh-switch-cost` plugin addresses the issue that DeepSeek Harness users cannot easily compare the cost of the active route against alternative routes during model inference. It performs local calculations to quantify cost differences between the current actual route and all alternative routes without manual estimation. The plugin relies on the `sessionProjections` projection registry and supports time-based billing, cache pricing, and route comparison. It exports a price list via the `/prices` endpoint and registers a `switch_cost` tool for the Agent to call, displaying per-model costs and percentage differences. When using it, note the source of token counts and that pricing data is only a snapshot.