feat(llm): add OpenRouter API support with proper headers and API key handling

- Add explicit OPENROUTER_API_KEY environment variable handling
- Add HTTP-Referer and X-Title headers for OpenRouter attribution
- Fix case sensitivity for provider names (ollama now case-insensitive)
- Add embedding fallback to OpenAI when using OpenRouter (since OpenRouter lacks embedding API)
- Add comprehensive test suite (30 tests) for OpenRouter integration
- Update README.md and PROJECT.md with OpenRouter configuration docs
- Add CHANGELOG.md documenting the changes

Patterns borrowed from ~/.claude/lib/genai_validate.py for multi-provider support.

Closes #1

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>