test+docs: lift upstream skill's before/after pairs as migration fixtures

[lex00]

The upstream skill gitlab-org/ci-cd/github-actions-to-gitlab-ci ships ~48 curated before/after pairs across its two reference files:

• references/syntax-mapping.md — ~15 pairs covering triggers, jobs/stages, steps/scripts, runs-on, matrix, env/secrets, containers/services, artifacts, caching, conditional execution, reusable workflows, job control (timeout/continue-on-error/concurrency/permissions), outputs, workflow_dispatch inputs, and github.* context expressions
• references/marketplace-actions.md — 33 pairs, one per Tier 1/2/3 action

This is canonical migration material curated by the GitLab CI team. Lifting it into chant gives us the test corpus, the regression-assertion fixtures, and the docs demos as a single dataset, aligned 1:1 with the ActionMapping[] entries from #87 + #88.

Layout:

lexicons/gitlab/src/migrate/from-github/fixtures/
  syntax-mapping/
    triggers/
      input.yml                 # the GH workflow from the skill's before block
      expected.gitlab-ci.yml    # the GL pipeline from the skill's after block
      expected-report.sarif     # provenance the migration tool is expected to emit
    matrix/
    env-secrets/
    ...
  marketplace-actions/
    tier-1/
      actions-checkout/
      actions-setup-node/
      ...
    tier-2/
    tier-3/

Tests under migrate.test.ts iterate the fixture tree and assert chant migrate input.yml produces the expected YAML (canonicalised compare) and the expected SARIF provenance. A curated subset embeds in #94's migration.mdx as worked examples (don't render all 48 — pick ~6 representative ones).

Done when

• Every before/after pair from the upstream skill's two reference files exists as a fixture directory with input.yml + expected.gitlab-ci.yml.
• Each fixture has a corresponding expected-report.sarif capturing the provenance the migration tool emits.
• migrate.test.ts iterates the fixture tree and asserts a match for each.
• A curated subset (~6 fixtures) renders inline in migration.mdx (docs: GitLab lexicon migration page #94).
• LICENSE-3RD-PARTY (or equivalent) credits the upstream skill's MIT license for the lifted fixture content.

Depends on: #85, #86, #87, #88. Blocks: #94.

Inspired by

gitlab-org/ci-cd/github-actions-to-gitlab-ci — references/syntax-mapping.md and references/marketplace-actions.md. Both files are MIT-licensed; their before/after blocks are the exact fixture content this issue ports.

[lex00]

Implemented in f9543c6 on feat/migrate-github-to-gitlab. PR #99 is open (draft); CI green (check / test / validate / link-check all pass). Awaiting merge.