Add xAI's Grok as a new web_search provider alongside Brave and Perplexity.
Uses the xAI /v1/responses API with tools: [{type: "web_search"}].
Configuration:
- tools.web.search.provider: "grok"
- tools.web.search.grok.apiKey or XAI_API_KEY env var
- tools.web.search.grok.model (default: grok-4-1-fast)
- tools.web.search.grok.inlineCitations (optional, embeds markdown links)
Returns AI-synthesized answers with citations similar to Perplexity.
