Add Exa AI as a third web search provider alongside Brave and Perplexity.
Exa is a search API built for AI agents with optional page text extraction.
Config:
- provider: "exa"
- exa.apiKey (or EXA_API_KEY env var)
- exa.contents (default: true) - include page text
- exa.maxChars (default: 1500) - chars per result
