Enhancement: make custom ACP model routes easier to select for Magpie #1098

Context

When configuring the DeepSeek Harness agent in Codeg to use an OpenAI-compatible Magpie gateway, the provider endpoint and credentials can be entered in the agent settings. Magpie exposes the routing group group/deepseek, while deepseek-acp defaults to deepseek-flash.

The existing environment-variable field makes this work by setting:

DEEPSEEK_ACP_MODEL=group/deepseek

This is therefore not a blocking configuration bug. The request is for a more discoverable model selection experience in Codeg.

Observed behavior

1. Configure the DeepSeek Harness agent with a Magpie endpoint and credentials.
2. Start a new session without a model override.
3. Magpie rejects the first request because it does not serve the ACP default deepseek-flash; its available route is group/deepseek.
4. Add DEEPSEEK_ACP_MODEL=group/deepseek in the existing environment-variable field; the session then works successfully.

Suggested improvement

• Add a dedicated default-model field or selector next to the provider endpoint and API key.
• For a custom OpenAI-compatible endpoint, optionally load model choices from its /v1/models response.
• Add a short hint or example for DEEPSEEK_ACP_MODEL in the DeepSeek Harness setup panel.

Environment

• Windows
• Codeg DeepSeek Harness agent
• deepseek-acp 0.9.0
• Magpie OpenAI-compatible gateway
• Working route: group/deepseek