dsh-reasoning-settings: Add a Reasoning Intensity Settings Page for DSH Custom llm-pi-ai Providers

`dsh-reasoning-settings` is a MIT-licensed plugin for DSH, primarily used to configure the reasoning strength of custom `llm-pi-ai` providers. This plugin addresses two core issues: firstly, it allows users to declare reasoning strength levels from `off` to `max` for custom models, defining the actual wire value sent and the provider's default values; secondly, it optimizes subagent invocations to correctly inherit the parent's provider, model, and reasoning strength, avoiding the use of incorrect official default settings. The plugin provides a settings page to automatically read added providers and models, enabling secure parameter configuration via the official API without touching API keys. It extends the `subagent` and `subagent_for` functionality.