feat(mcp): restore Exa websearch support (#549)

* feat(mcp): restore Exa MCP websearch support

- Add websearch.ts with Exa remote MCP configuration
- Update McpNameSchema to include websearch
- Wire websearch MCP into plugin initialization

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

* test(mcp): update tests and docs for websearch MCP

- Update index.test.ts to verify 3 MCPs (websearch, context7, grep_app)
- Add Exa/websearch documentation to README.md MCPs section

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/mcp/types.ts

@@ -1,6 +1,6 @@
 import { z } from "zod"
 
-export const McpNameSchema = z.enum(["context7", "grep_app"])
+export const McpNameSchema = z.enum(["websearch", "context7", "grep_app"])
 
 export type McpName = z.infer<typeof McpNameSchema>