feat: extract cms from mp/cms — initial libreshop/cms

Source moved verbatim from mp/cms/ on 2026-04-29; mp was the first
concrete adapter consuming the libreshop toolkit. Builds and publishes
git.librete.ch/libreshop/cms on every main / v* push via the standard
.gitea/workflows/build.yml shared across libreshop components.

README.md

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+# libreshop/cms
+
+Strapi-based content backend (catalogue, orders, customers).
+
+Part of the [libreshop](https://git.librete.ch/libreshop) toolkit. Image
+published at `git.librete.ch/libreshop/cms` on every push to `main`
+and on `v*` tags.
+
+## Source
+
+This repo was extracted from `mp/cms/` on 2026-04-29; mp was the
+first concrete adapter consuming the toolkit. mp's `compose.yml` now
+pulls `git.librete.ch/libreshop/cms:<pin>` instead of building locally.
+
+## Build locally
+
+```
+docker build -t libreshop/cms:dev .
+```
+
+## Adapter contract
+
+See `docker-entrypoint.sh` and `Dockerfile` for the runtime surface.
+Adapters configure the component via env vars and bind-mounted volumes;
+do not patch the running container or rely on internal paths.