dsh-subagent-toolkit: Specify third-party models for individual DSH sub-agents

By default, the subagent in DeepSeek Harness follows the main model, and integrating third-party models often requires a complete replacement of the main model. The dsh-subagent-toolkit (maintained by leotpp, an inference tool, v0.2.0, MIT) enables adding OpenAI-compatible channels, fetching model lists, and fixing a unique subagent via the 'Settings → Submodel' page. It writes configurations to `subagent-model-selection-settings` and `llm-pi-ai.providers`, and utilizes a system prompt to instruct the main Agent to explicitly pass parameters when exactly one route is active. The plugin does not modify the DSH kernel; optionally, `subagent_pi` can utilize the official Pi CLI. Note that the fixed subagent is a soft constraint, applies only to new sessions, relies on DSH extension points, and may become invalid after upgrades.