feat(mcp): add websearch_exa as built-in MCP

- Add src/mcp/ directory with websearch_exa configuration
- Configure Exa AI remote MCP (https://mcp.exa.ai/mcp)
- Export builtinMcps from plugin config
- Update documentation (EN/KO) with Built-in MCPs section

src/mcp/index.ts

@@ -0,0 +1,5 @@
+import { websearch_exa } from "./websearch-exa"
+
+export const builtinMcps = {
+  websearch_exa,
+}