feat: chant migrate command (GitHub Actions → GitLab CI/CD)

[lex00]

Tracking issue for a typed, IR-driven migration tool from GitHub Actions to GitLab CI/CD, replacing the freehand-LLM approach that gitlab-org/ci-cd/github-actions-to-gitlab-ci uses today.

Both lexicons already ship YAML parsers, IR, code generators, typed constructors, and lint rules. This work wires them together: GH yaml → GH IR → GitLab IR (new transformer) → GitLab yaml or chant TS, with per-node provenance recorded as SARIF.

The end-state CLI:

npx @intentius/chant-lexicon-gitlab migrate \
  .github/workflows/ci.yml \
  --emit yaml --output .gitlab-ci.yml \
  --report migration-report.sarif

Sub-issues

• feat: GitHub→GitLab IR transformer #85 — GitHub→GitLab IR transformer
• feat: chant migrate CLI (--emit yaml) #86 — chant migrate CLI (--emit yaml)
• feat: ActionMapping registry + Tier 1 mappings #87 — ActionMapping registry + Tier 1 mappings
• feat: ActionMapping Tier 2 + Tier 3 coverage #88 — ActionMapping Tier 2 + Tier 3 coverage
• feat: chant migrate --emit ts #89 — --emit ts (TypeScript output)
• feat: chant migrate SARIF report + --strict #90 — SARIF report + --strict
• feat: chant migrate --validate (glci/glab) #91 — --validate (glci/glab integration)
• feat: chant migrate --use-composites (Node patterns) #92 — --use-composites pattern recognisers (Node first)
• test: roundtrip harness for chant migrate #93 — roundtrip harness
• docs: GitLab lexicon migration page #94 — docs page: migration.mdx + inspiration section
• test+docs: lift upstream skill's before/after pairs as migration fixtures #98 — lift upstream skill's before/after pairs as migration fixtures
• feat: chant-gitlab-migrate skill #95 — chant-gitlab-migrate skill
• feat: gitlab MCP server — migrate tool #96 — gitlab MCP tool: migrate

Deferred to follow-ups: composite actions (.github/actions/*/action.yml), reusable workflows with inputs:/outputs:, multi-file workflow trees, reverse direction (GitLab→GitHub), extraction to @intentius/chant-migrate.

Inspired by

• gitlab-org/ci-cd/github-actions-to-gitlab-ci — https://gitlab.com/gitlab-org/ci-cd/github-actions-to-gitlab-ci. The four-category provenance model (Translated/Approximated/NeedsReview/Lost), the tiered marketplace-action mapping, the syntax-mapping reference, and the glci/glab validator-as-final-stage pattern are direct ports. The chant version is the same translation rules wearing a compiler instead of a prompt.
• GitLab's official migration guide — https://docs.gitlab.com/ci/migration/github_actions/.

[lex00]

All 13 sub-issues closed; checklist ticked. PR #99 open (draft) with CI green. Awaiting merge.