Add Rails example app

## Motivation and Context

The README.md documents the "Rails (mount)" integration pattern, but the repository
had no runnable Rails example. This adds a minimal Rails application under examples/rails
that mounts StreamableHTTPTransport at /mcp and demonstrates class-based tools and
a resource with a read handler.

The MCP server is built in config/routes.rb rather than in an initializer because
Zeitwerk sets up autoloading after initializers run, so the tool classes in app/tools
cannot be referenced there.
Code reloading is disabled because the transport holds the server built at boot.
The app is booted with puma instead of rackup because rackup inserts `Rack::Lint`
in its development default, which rejects the capitalized response header names
that the transport currently emits.

Closes #44.

## How Has This Been Tested?

The curl walkthrough in examples/rails/README.md has been verified end to end against
the running app: initialize and the initialized notification, tools/list, tools/call for
both tools, resources/list, resources/read, the standalone SSE stream,
and session termination via DELETE.

## Breaking Changes

None. This only adds an example application and documentation links.

examples/rails/config/application.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../Gemfile", __dir__)
+
+require "bundler/setup"
+
+require "rails"
+require "action_controller/railtie"
+
+require "mcp"
+
+module McpRailsExample
+  class Application < Rails::Application
+    config.load_defaults(8.0)
+
+    # This example has no views, assets, or database.
+    config.api_only = true
+
+    # The MCP server is built once at boot (config/routes.rb) and holds references to the tool classes in app/tools,
+    # so code reloading is disabled; restart the server after changing tool or resource code.
+    # A reloading app would need `config.to_prepare` and a way to swap the transport's server instead.
+    config.enable_reloading = false
+    config.eager_load = true
+
+    config.logger = ActiveSupport::Logger.new($stdout)
+  end
+end