feat: ActionMapping registry + Tier 1 mappings

[lex00]

Today the transformer (#85) drops every uses: step it doesn't recognise into a NeedsReview provenance record. This issue adds a typed registry so the 14 most-used marketplace actions translate cleanly.

type ActionMapping = {
  uses: string;                      // "actions/checkout"
  majorVersions?: string[];          // ["v3", "v4"] — match all if omitted
  apply: (step: GitHubStep, ctx: MigrationContext) => MappingResult;
};

type MappingResult =
  | { kind: "delete"; reason: string }
  | { kind: "image"; image: GitLabImage }      // job-level image substitution
  | { kind: "script"; lines: string[] }
  | { kind: "service"; service: GitLabService }
  | { kind: "needsReview"; reason: string };

Tier 1 (from the upstream skill's references/marketplace-actions.md):

Action	Mapping
actions/checkout	delete
actions/setup-node	image: node:<v>
actions/setup-python	image: python:<v>
actions/setup-java	image: eclipse-temurin:<v>
actions/setup-go	image: golang:<v>
actions/setup-ruby	image: ruby:<v>
actions/cache	cache: keyword (handled at job level)
actions/upload-artifact	artifacts: keyword
actions/download-artifact	needs:artifacts: or removed when adjacent stages
docker/build-push-action	script + services: docker:dind
docker/login-action	script (docker login)
docker/setup-buildx-action	script + services: docker:dind
docker/setup-qemu-action	script (docker run tonistiigi/binfmt)
actions/github-script	needsReview

Done when

• lexicons/gitlab/src/migrate/from-github/action-mappings.ts exports an ActionMapping[] covering all 14.
• Each mapping has a unit test with before/after fixtures.
• Transformer (feat: GitHub→GitLab IR transformer #85) consults the registry; misses produce a needsReview provenance record with the action name.
• Action-level mappings can return job-level effects (image, services, cache) — the transformer applies these correctly.

Depends on: #85. Blocks: #88.

Inspired by

gitlab-org/ci-cd/github-actions-to-gitlab-ci — references/marketplace-actions.md Tier 1 section. Each chant ActionMapping is a direct port of that file's before/after examples.

[lex00]

Implemented in 9f60e14 on feat/migrate-github-to-gitlab. PR #99 is open (draft); CI green (check / test / validate / link-check all pass). Awaiting merge.