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/Gemfile

@@ -0,0 +1,10 @@
+source "https://rubygems.org"
+
+gem "rails", "~> 8.0.0"
+gem "mysql2", "~> 0.5"
+gem "puma"
+
+# In this example stack the engine comes straight from the repo checkout the
+# compose file mounts at /warp_engine. A real host would use the git source or
+# the Forgejo rubygems registry instead — see the engine README.
+gem "warp_engine", path: ENV.fetch("WARP_ENGINE_PATH", "/warp_engine")