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

examples/rails/config/environment.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+require_relative "application"
+
+Rails.application.initialize!

examples/rails/config/routes.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+# The MCP server and its Streamable HTTP transport are built here, once at boot,
+# following the "Rails (mount)" pattern from the top-level README.
+# Routes are loaded after the application initializes, so the tool classes in app/tools
+# are available at this point (they are not yet available in config/initializers,
+# where Zeitwerk has not set up autoloading).
+#
+# The transport keeps session and SSE state in memory, so run a single server process
+# (e.g., Puma with `workers 0`, which is the rackup default).
+# See the top-level README for multi-instance and stateless deployment notes.
+server = MCP::Server.new(
+  name: "rails_example_server",
+  title: "Rails Example Server",
+  version: "1.0.0",
+  tools: [AddTool, GreetingTool],
+  resources: [
+    MCP::Resource.new(
+      uri: "example://rails/readme",
+      name: "readme",
+      title: "Example README",
+      description: "Describes this Rails example MCP server",
+      mime_type: "text/plain",
+    ),
+  ],
+  server_context: { app_name: "mcp_rails_example" },
+)
+
+server.resources_read_handler do |params|
+  [
+    {
+      uri: params[:uri],
+      mimeType: "text/plain",
+      text: "This resource is served by the Rails example MCP server (Rails #{Rails.version}).",
+    },
+  ]
+end
+
+transport = MCP::Server::Transports::StreamableHTTPTransport.new(server)
+
+Rails.application.routes.draw do
+  # `StreamableHTTPTransport` is a Rack app, so it can be mounted directly.
+  # `mount` directs all HTTP methods on /mcp to the transport, which internally dispatches POST (JSON-RPC messages),
+  # GET (SSE stream), and DELETE (session termination) per the MCP Streamable HTTP transport spec.
+  mount(transport => "/mcp")
+end