feat: chant-gitlab-migrate skill

[lex00]

A new skill at lexicons/gitlab/src/skills/chant-gitlab-migrate.md, installed via chant init --lexicon gitlab --skill migrate to .claude/skills/ (and the cross-tool ~/.agents/skills/ path).

The skill's job is operational glue, not translation logic:

• Recognise when the user wants to migrate (triggers: "migrate from GitHub Actions", "convert this workflow", paste of .github/workflows/*.yml, etc. — same trigger phrases as the upstream skill)
• Shell out to npx @intentius/chant-lexicon-gitlab migrate ...
• Surface the Markdown report and any NeedsReview items to the user
• Suggest GitLab-only upgrade moments the recogniser didn't catch (DAG needs:, rules:changes:, child pipelines, include:)

Frontmatter and section structure mirror the existing chant-gitlab.md skill.

Done when

• chant-gitlab-migrate.md lands.
• Listed in lexicons/gitlab/src/skills/ and shipped in the package's dist/skills/.
• chant init --lexicon gitlab --skill migrate writes it to .claude/skills/chant-gitlab-migrate/SKILL.md.
• skills.mdx (docs: GitLab lexicon migration page #94) documents it.

Depends on: #86.

Inspired by

gitlab-org/ci-cd/github-actions-to-gitlab-ci/SKILL.md — the trigger-phrase patterns, the four-category report shape, and the "suggest GitLab improvements" step are direct ports. The difference: this skill calls a compiler instead of being one.

[lex00]

Implemented in dad39c7 on feat/migrate-github-to-gitlab. PR #99 is open (draft); CI green (check / test / validate / link-check all pass). Awaiting merge.