fix: add EXA_API_KEY header support for websearch_exa MCP (#499)

The remote MCP endpoint https://mcp.exa.ai/mcp requires API key authentication
via x-api-key header. Without this, the connection times out waiting for auth.

This change:
- Reads EXA_API_KEY from environment variable
- Passes it as x-api-key header when available
- Updates RemoteMcpConfig type to support optional headers

Co-authored-by: Junho Yeo <i@junho.io>

src/mcp/websearch.ts

@@ -2,4 +2,7 @@ export const websearch = {
   type: "remote" as const,
   url: "https://mcp.exa.ai/mcp?tools=web_search_exa",
   enabled: true,
+  headers: process.env.EXA_API_KEY
+    ? { "x-api-key": process.env.EXA_API_KEY }
+    : undefined,
 }