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/index.ts

@@ -1,3 +1,4 @@
+import { websearch } from "./websearch"
 import { context7 } from "./context7"
 import { grep_app } from "./grep-app"
 import type { McpName } from "./types"
@@ -5,6 +6,7 @@ import type { McpName } from "./types"
 export { McpNameSchema, type McpName } from "./types"
 
 const allBuiltinMcps: Record<McpName, { type: "remote"; url: string; enabled: boolean }> = {
+  websearch,
   context7,
   grep_app,
 }