WIP - OpenRouter support and related refactoring

docs/openrouter.md

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+# OpenRouter Setup
+
+OpenRouter provides unified access to multiple AI models (GPT-4, Claude, Mistral, etc.) through a single API.
+
+## Quick Start
+
+### 1. Get API Key
+1. Sign up at [openrouter.ai](https://openrouter.ai/)
+2. Create an API key from your dashboard
+3. Add credits to your account
+
+### 2. Set Environment Variable
+```bash
+# Add to your .env file
+OPENROUTER_API_KEY=your-openrouter-api-key
+```
+
+That's it! Docker Compose already includes all necessary configuration.
+
+### 3. Use Any Model
+```
+# Examples
+"Use gpt-4 via zen to review this code"
+"Use claude-3-opus via zen to debug this error"
+"Use mistral-large via zen to optimize this algorithm"
+```
+
+## Cost Control (Recommended)
+
+Restrict which models can be used to prevent unexpected charges:
+
+```bash
+# Add to .env file - only allow specific models
+OPENROUTER_ALLOWED_MODELS=gpt-4,claude-3-sonnet,mistral-large
+```
+
+Check current model pricing at [openrouter.ai/models](https://openrouter.ai/models).
+
+## Available Models
+
+Popular models available through OpenRouter:
+- **GPT-4** - OpenAI's most capable model
+- **Claude 3** - Anthropic's models (Opus, Sonnet, Haiku)
+- **Mistral** - Including Mistral Large
+- **Llama 3** - Meta's open models
+- Many more at [openrouter.ai/models](https://openrouter.ai/models)
+
+## Troubleshooting
+
+- **"Model not found"**: Check exact model name at openrouter.ai/models
+- **"Insufficient credits"**: Add credits to your OpenRouter account
+- **"Model not in allow-list"**: Update `OPENROUTER_ALLOWED_MODELS` in .env