feat(mcp): add grep_app builtin MCP for ultra-fast GitHub code search

- Add grep_app MCP configuration (https://mcp.grep.app)
- Update explore agent with grep_app usage guide:
  - Always launch 5+ grep_app calls with query variations
  - Always add 2+ other search tools for verification
  - grep_app is fast but potentially outdated, use as starting point only
- Update README.md and README.ko.md with grep_app documentation

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

src/mcp/types.ts

@@ -1,5 +1,5 @@
 import { z } from "zod"
 
-export const McpNameSchema = z.enum(["websearch_exa", "context7"])
+export const McpNameSchema = z.enum(["websearch_exa", "context7", "grep_app"])
 
 export type McpName = z.infer<typeof McpNameSchema>