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/.env.example

@@ -0,0 +1,24 @@
+# Copy to .env and adjust. Every value except the OAuth pair has a working
+# default for a throwaway local demo, so the quickstart runs without edits.
+
+MYSQL_ROOT_PASSWORD=warpengine
+
+# Shared secret for the /update endpoint (X-Update-Secret header).
+UPDATE_SECRET=example-update-secret
+
+# Password of the "drop" user on the artifact drop area (SSH, port 2222).
+DROP_PASSWORD=drop
+
+# Published ports.
+APP_PORT=8080
+DROPAREA_SSH_PORT=2222
+GITEA_SSH_PORT=2223
+
+# --- profile "ci" only -------------------------------------------------------
+
+WOODPECKER_AGENT_SECRET=example-agent-secret
+
+# OAuth2 application created in gitea (Settings -> Applications), redirect URI
+# http://woodpecker:8000/authorize — required before the ci profile starts.
+WOODPECKER_GITEA_CLIENT=
+WOODPECKER_GITEA_SECRET=