Integrate Exa (semantic search) and Firecrawl (deep web scraping) as
MCP-based research tools, following the existing Brave Search pattern.
- Add tool declarations to all 3 researcher agents
- Add tool strategy sections with usage guidance and priority
- Add config detection for FIRECRAWL_API_KEY and EXA_API_KEY env vars
- Add firecrawl/exa_search config keys to core defaults and init output
- Update source priority hierarchy across all researchers
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