Add the compose_stack role, inventory and playbooks

One role that syncs a payload and brings the stack up, configured entirely
through stack_* variables, plus six playbooks that each demonstrate one part
of that contract. Every playbook runs standalone and is tagged with its
stack name, so site.yml --tags <stack> works.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

ansible/roles/compose_stack/tasks/main.yml

@@ -0,0 +1,20 @@
+---
+# Two entry points, so a playbook that must do maintenance between the payload
+# landing and the stack starting can interleave its own tasks:
+#
+#   - ansible.builtin.include_role:
+#       name: compose_stack
+#       tasks_from: sync
+#   - name: Run the database migration
+#     ...
+#   - ansible.builtin.include_role:
+#       name: compose_stack
+#       tasks_from: up
+#
+# Using the role normally (roles: - role: compose_stack) runs both, in order.
+# playbooks/webapp.yml is the worked example.
+- name: Sync the payload to the host
+  ansible.builtin.include_tasks: sync.yml
+
+- name: Start the stack
+  ansible.builtin.include_tasks: up.yml