Reduce release package size

The current release package contains files that are not required at runtime,
such as `.github`, `docs`, and `examples`.

```ruby
Dir.chdir(File.expand_path("..", __FILE__)) do
  %x(git ls-files -z).split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
end
=>
[".gitattributes",
 ".github/dependabot.yml",
 ".github/workflows/ci.yml",
 ".github/workflows/release.yml",
 ".gitignore",
 ".rubocop.yml",
 "AGENTS.md",
 "CHANGELOG.md",
 "CODE_OF_CONDUCT.md",
 "Gemfile",
 "LICENSE",
(snip)
```

This PR removes unnecessary files where possible to reduce the size of the release package.

Author: koic

mcp.gemspec

@@ -23,9 +23,7 @@ Gem::Specification.new do |spec|
   spec.metadata["bug_tracker_uri"] = "#{spec.homepage}/issues"
   spec.metadata["documentation_uri"] = "https://rubydoc.info/gems/mcp"
 
-  spec.files = Dir.chdir(File.expand_path("..", __FILE__)) do
-    %x(git ls-files -z).split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
-  end
+  spec.files = Dir["LICENSE", "README.md", "lib/**/*"]
 
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }