feat: add Exa as default web search provider

[kesku]

Related to #2664

Summary

web_search (inside agentic_fetch) currently scrapes DDG and is pretty flaky! This makes Exa the default via its free hosted MCP (https://mcp.exa.ai/mcp) with no signup or API key required

Engine selection is configured in crush.json:

{
  "tools": {
    "web_search": {
      "search_engine": "exa",
      "exa_api_key": "$EXA_API_KEY"
    }
  }
}

• EXA_API_KEY is optional if you need higher rate limits otherwise it's free without API key
• this keeps DDG as both an explicit config choice and the automatic fallback when Exa fails

Testing

• go test ./internal/agent/tools ./internal/config ./internal/cmd
• go test -tags integration ./internal/agent/tools -run TestExaHostedIntegration -v
• Local testing:

[charmcli]

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[kesku]

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