Add OpenRouter as a provider (access to 100+ models)

[xmaks82]

Description

Add OpenRouter as a provider, giving access to 100+ models through a single API key.

Implementation

1. Create src/agent/clients/openrouter_client.py extending BaseLLMClient
2. OpenRouter uses the OpenAI-compatible API format
3. Add popular model mappings to factory.py
4. Handle the HTTP-Referer and X-Title headers that OpenRouter recommends

Reference

• OpenRouter API docs
• Free tier models available