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/greeting_tool.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class GreetingTool < MCP::Tool
+  title "Greeting Tool"
+  description "Greets the given name and reports which app served the request"
+  input_schema(
+    properties: {
+      name: { type: "string" },
+    },
+    required: ["name"],
+  )
+  annotations(
+    destructive_hint: false,
+    idempotent_hint: true,
+    open_world_hint: false,
+    read_only_hint: true,
+  )
+
+  class << self
+    def call(name:, server_context:)
+      MCP::Tool::Response.new([{
+        type: "text",
+        text: "Hello, #{name}! (served by #{server_context[:app_name]})",
+      }])
+    end
+  end
+end