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/initializers/apipie.rb

@@ -0,0 +1,11 @@
+Apipie.configure do |config|
+  config.app_name = "WarpEngine Example Host"
+  config.api_base_url = ""
+  config.doc_base_url = "/api/docs"
+  config.api_controllers_matcher = [
+    "#{WarpEngine::Engine.root}/app/controllers/**/*.rb"
+  ]
+  config.validate = false
+  config.translate = false
+  config.default_version = "1.0"
+end