Fix package root entrypoint

[alejandrorivas-pixel]

Summary

• add a root index.js entrypoint that exports createMcpApp() and JSON-RPC response helpers
• refactor bin/server.js so the CLI still starts the listener, while importing mcp-server does not
• add node:test coverage for the root export and initialize capabilities response
• refresh the npm lockfile metadata to match the current package version/bin

Fixes #2.

Validation

• npm test
• git diff --check
• packed tarball smoke: install local tarball and import('mcp-server') returns a createMcpApp factory