Add a runnable example compose stack for WarpEngine

examples/compose boots everything the engine's workflow assumes: mysql, a
minimal Rails host consuming the engine as a path gem, an SSH drop area
sharing the softwares volume with the app, and — behind the ci profile —
gitea plus woodpecker (agent attached to the stack network so pipeline
steps reach droparea/app by service name).

host_app doubles as a reference for a brand-new host: Gemfile, the two
initializers, apipie + engine mounts in routes.rb; on first boot the
entrypoint runs the install generator and db:prepare.

The README gains a detailed bring-up walkthrough: quickstart, publishing
a release by hand over scp + /update (verified end to end from a clean
slate), and the full gitea/woodpecker OAuth wiring.

examples/compose/host_app/config/application.rb

@@ -0,0 +1,23 @@
+require_relative "boot"
+
+require "rails"
+require "active_model/railtie"
+require "active_record/railtie"
+require "action_controller/railtie"
+require "action_view/railtie"
+require "action_dispatch/railtie"
+
+Bundler.require(*Rails.groups)
+require "warp_engine"
+
+module HostApp
+  class Application < Rails::Application
+    config.load_defaults 8.0
+
+    config.time_zone = "UTC"
+    config.active_record.default_timezone = :utc
+
+    # Demo stack: reachable as localhost, gitea-network hostnames, etc.
+    config.hosts.clear
+  end
+end