AD-25 Add OpenRouter Fusion support for multi-model deliberation

[Finfinder]

Objective

Integrate OpenRouter Fusion to enable multi-model deliberation where the agent sends queries to multiple models and synthesizes their responses.

Context

OpenRouter Fusion is a server-side tool that enables multi-model deliberation. When invoked, it sends the same prompt to multiple models (the panel), analyzes their responses for consensus, contradictions, and unique insights, then synthesizes a final answer using a judge model. This improves response quality for complex queries by leveraging diverse model perspectives.

Scope

• Add Fusion configuration to Settings Service: enable/disable flag, panel models list, judge model selection
• Implement Fusion adapter in Model Gateway that uses openrouter/fusion model alias
• Support Fusion plugin configuration with custom analysis_models and model (judge) parameters
• Handle Fusion response format: extract analysis.consensus, analysis.contradictions, analysis.unique_insights, and individual responses
• Add Fusion toggle in Chat tab UI for user control
• Implement fallback to single-model mode when Fusion is disabled or fails
• Document Fusion usage and configuration in docs/model-gateway.md

Fusion API Details

• Model alias: openrouter/fusion
• Plugin configuration:

  {
    "model": "openrouter/fusion",
    "plugins": [{
      "id": "fusion",
      "analysis_models": ["~anthropic/claude-opus-latest", "~openai/gpt-latest"],
      "model": "~openai/gpt-latest"
    }]
  }

• Response includes structured analysis with consensus, contradictions, and raw panel responses

Rationale

Multi-model deliberation can significantly improve answer quality for complex queries by aggregating diverse model perspectives. Fusion provides this capability server-side without requiring client-side orchestration.

Benefits

• Higher quality responses for complex/ambiguous queries
• Automatic detection of model disagreements and blind spots
• Transparent synthesis showing which models contributed which insights
• Configurable panel composition for different use cases

Definition of Done

• Fusion configuration exists in Settings Service with panel models and judge model options
• Model Gateway adapter supports openrouter/fusion with plugin configuration
• Chat tab UI includes Fusion toggle control
• Fusion responses are properly parsed and displayed
• Fallback to single-model mode works when Fusion is disabled or fails
• Documentation in docs/model-gateway.md covers Fusion configuration and usage

Milestone

1.0 Stabilize MVP