ci: add Codeberg CI (Woodpecker + Forgejo) and document CI/CD topology

Add a lightweight test-fast pipeline for the public Codeberg mirror so it
shows a build status, defined for both CI systems Codeberg offers:
- .woodpecker.yml            (Woodpecker, debian:bookworm)
- .forgejo/workflows/        (Forgejo Actions, node:20-bookworm, runner label docker)

Both run the pure-math suite only (Eigen vendored, GTest via FetchContent;
no CGAL/Boost), using public images and the docker runner label since the
private eulernest ci-cpp image and Pi runner are unreachable from Codeberg.

Docs:
- new doc/architecture/ci-cd.md: two-forge topology, runners, trigger
  keywords, mirror mechanism.
- fix stale "codeberg/main ... no CI" claim in contributing.md and bring
  the CI table up to date.
- link the new doc from the README doc index.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

doc/contributing.md

@@ -13,11 +13,12 @@ with English.
 ## Git workflow
 
 - `main` is protected on `origin` (Gitea). Push to `dev`, then open a pull request.
-- `codeberg/main` can be pushed to directly (public mirror, no CI).
+- `codeberg/main` is the public mirror. It now also runs CI of its own
+  (Woodpecker + Forgejo Actions) — see [architecture/ci-cd.md](architecture/ci-cd.md).
 - Both remotes must stay in sync after every significant change:
   ```bash
-  git push origin HEAD:dev   # triggers CI
-  git push codeberg main     # updates public mirror
+  git push origin HEAD:dev   # triggers eulernest CI (full suite, keyword-gated CGAL)
+  git push codeberg main     # updates public mirror → triggers Codeberg CI (test-fast)
   ```
 - Branch naming: `feature/<topic>`, `fix/<topic>`, `phase<N>-<topic>`
 
@@ -25,17 +26,30 @@ with English.
 
 ## CI
 
-Two jobs run on push to `dev`/`main` or on pull requests:
+The project runs CI on **two forges**. Full topology, runners, images and
+trigger keywords are documented in [architecture/ci-cd.md](architecture/ci-cd.md).
+
+**eulernest Gitea** (`origin`, self-hosted Raspberry Pi / ARM64) — the
+authoritative gate. On push to `dev`/`main` or a pull request:
 
 | Job | What it tests | Trigger |
 |---|---|---|
 | `test-fast` | pure-math tests, no Boost | all branches |
-| `test-cgal` | full CGAL test suite | `main`, `dev`, PRs only |
+| `test-cgal` | full CGAL test suite | commit message contains `/test-cgal` |
+| `quality-gates` | license / codespell / shellcheck / CGAL conventions | commit message contains `/quality-gates` |
 
-A PR is ready to merge when both jobs pass.
+A PR is ready to merge when `test-fast` (and, for CGAL-touching work,
+`test-cgal`) pass. See `.gitea/workflows/cpp-tests.yml` and
+`.gitea/docker/Dockerfile.ci-cpp`.
 
-The runner is a self-hosted Raspberry Pi (ARM64). See `.gitea/workflows/cpp-tests.yml`
-and `.gitea/docker/Dockerfile.ci-cpp`.
+**Codeberg** (`codeberg`, public mirror, shared runners) — a lightweight
+`test-fast` mirror so the public repo also shows a build status. Defined twice,
+once per CI system Codeberg offers:
+
+| File | CI system |
+|---|---|
+| `.woodpecker.yml` | Woodpecker CI (ci.codeberg.org) |
+| `.forgejo/workflows/cpp-tests.yml` | Forgejo Actions |
 
 ---