docs(adr-0028): accept CI/CD + git hosting migration to GitLab

ADR-0028 shipped as proposed in #226. No open questions on drivers,
considered options, or the 4-phase migration sequence - promote to
accepted to unblock Phase 1 (mirror-and-bootstrap).

ADR-0015 gets a status-update blockquote at the top calling out that
the platform choice "Gitea Actions" is superseded by ADR-0028 while
the architectural principles in the rest of ADR-0015 (trunk-based +
squash, all-gates-blocking, thin YAML, on-prem runners, signed
commits, Conventional Commits) carry over unchanged. ADR-0015's
frontmatter status stays accepted - this is the canonical MADR way
to handle partial supersession without rewriting the decision record.

CLAUDE.md CI/CD architecture bullet rewritten to reflect the
accepted-but-not-yet-implemented state. Roll-up bumped to 0001 -> 0028
accepted. Side-fix: the bullet referenced a non-existent `ci:scan`
script; replaced with the real names (ci:catalogue-drift, ci:audit,
ci:perf, ci:gzip-budgets).

No code changes, no ci:check impact.

docs/decisions/README.md

@@ -71,4 +71,4 @@ ADRs are listed in numerical order. To slice by topic, filter on the `Tags` colu
 | [0025](0025-authorization-model-privileges-roles-scopes.md)  | Authorization model — three orthogonal axes (privileges × functional roles × scopes), Entra-backed               | accepted | `security`, `backend`, `data`                                  | 2026-05-20 |
 | [0026](0026-person-user-portal-data-model.md)                | `Person` golden record + `User` portal-account — portal-side identity model                                      | accepted | `data`, `backend`, `security`                                  | 2026-05-24 |
 | [0027](0027-portal-side-organisational-hierarchy.md)         | Portal-side organisational hierarchy — `Structure` with kind discriminator and nullable FINESS / SIRET           | accepted | `data`, `backend`                                              | 2026-05-24 |
-| [0028](0028-migrate-cicd-and-git-hosting-to-gitlab.md)       | Migrate CI/CD + git hosting from Gitea to GitLab self-hosted                                                     | proposed | `infrastructure`, `process`                                    | 2026-05-26 |
+| [0028](0028-migrate-cicd-and-git-hosting-to-gitlab.md)       | Migrate CI/CD + git hosting from Gitea to GitLab self-hosted                                                     | accepted | `infrastructure`, `process`                                    | 2026-05-26 |