Support more open code models, including MIMO

[Cheggin]

Problem

The desktop app should support a broader set of open code models, including MIMO, so users have more options beyond the current configured engines/providers.

Scope

• Audit the current engine/provider registry and settings surfaces for where open code models should be added.
• Add support for additional open code models, with MIMO included in the first pass.
• Make model selection clear in the UI wherever users choose the code/agent engine.
• Define how credentials, base URLs, local endpoints, or provider-specific options should be configured for these models.
• Preserve existing Codex/Claude behavior while adding the new model options.

Acceptance criteria

• MIMO is available as a selectable code model/provider option where appropriate.
• The model can be configured and used for an agent run without breaking existing engine choices.
• Missing credentials or endpoint configuration produce actionable errors.
• Model/provider metadata is centralized enough that future open models can be added without duplicating UI and engine logic.
• Tests cover model registration, settings serialization, and at least one successful task-start path with the new model configuration mocked.