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/bin/docker-entrypoint

@@ -0,0 +1,16 @@
+#!/bin/sh
+set -e
+
+bundle check || bundle install
+
+# First boot: run the install generator the way a real host would. It creates
+# the create_warp_engine_tables migration; --skip leaves our initializer alone.
+if ! ls db/migrate/*create_warp_engine_tables* >/dev/null 2>&1; then
+  bin/rails generate warp_engine:install --skip
+fi
+
+bin/rails db:prepare
+
+rm -f tmp/pids/server.pid
+
+exec "$@"

examples/compose/host_app/bin/rails

@@ -0,0 +1,4 @@
+#!/usr/bin/env ruby
+APP_PATH = File.expand_path("../config/application", __dir__)
+require_relative "../config/boot"
+require "rails/commands"