docs(adr-0028): propose CI/CD + git hosting migration Gitea -> GitLab

ADR-0015 framed Gitea Actions as a level-2 implementation explicitly
slated for a GitLab migration within 6-18 months. That window opens
now: vm-gitlab is provisioned, the team is about to scale, and the
Gitea-specific friction points documented inline in .gitea/workflows/
ci.yml (paywalled gitleaks-action, github.com-bound trivy-action,
manual curl+tar install dance) are accumulating maintenance tax.

ADR-0028 records the platform shift. Architectural principles from
ADR-0015 (trunk-based + squash, all-gates-blocking, thin YAML over
portable scripts in pnpm + Nx, on-prem runners, Conventional Commits)
carry over unchanged - only host / pipeline-file grammar / runner type
/ scan tooling change. Decision-only; 4-phase migration (mirror,
parallel pipelines, cutover, cleanup) lands in follow-up PRs.

Renumbering: ADR-0028 was a placeholder reference for the future
Pleiades + Acteurs+ sync ADR. That ADR shifts to 0029; the 25 link
references across ADR-0026 and ADR-0027 update in lockstep. CLAUDE.md
roll-up adjusted accordingly.

Status: proposed. Accept will be a separate small PR per the established
pattern (see #219 for ADR-0026/0027 acceptance).

docs/decisions/README.md

@@ -71,3 +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 |