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/app/tools/add_tool.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class AddTool < MCP::Tool
+  title "Add Tool"
+  description "A simple example tool that adds two numbers"
+  input_schema(
+    properties: {
+      a: { type: "number" },
+      b: { type: "number" },
+    },
+    required: ["a", "b"],
+  )
+
+  class << self
+    def call(a:, b:)
+      MCP::Tool::Response.new([{
+        type: "text",
+        text: "The sum of #{a} and #{b} is #{a + b}",
+      }])
+    end
+  end
+end