examples: add with-mcp-servers

[Nishant-Chaudhary5338]

Summary

• Adds examples/with-mcp-servers — a pnpm Turborepo monorepo showing how to structure multiple Model Context Protocol (MCP) servers as isolated workspace packages
• Two MCP server packages: @repo/mcp-calculator (add, subtract, multiply, divide) and @repo/mcp-file-reader (read_file, list_directory), each using the official @modelcontextprotocol/sdk with stdio transport
• One client app: mcp-client — a Node.js CLI that spawns both servers and runs demo tool calls
• Turbo's ^build pipeline ensures server packages compile before the client; demonstrates cache hits on unchanged packages
• Zero dependencies beyond @modelcontextprotocol/sdk, typescript, and shared @repo/eslint-config / @repo/typescript-config

Closes #12938 (approved by @anthonyshew)

Test plan

npx create-turbo@latest --example with-mcp-servers
cd my-turborepo
pnpm install
pnpm build        # all 3 packages compile cleanly
pnpm check-types  # zero TypeScript errors
pnpm lint         # zero lint errors
node apps/mcp-client/dist/index.js
# 5 + 3 = 8
# 6 × 7 = 42
# Root directory: [...]
pnpm build        # second run → FULL TURBO (all tasks cached)

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