feat: ActionMapping Tier 2 + Tier 3 coverage

[lex00]

Extend the registry from #87 with the remaining 19 actions from the upstream skill's references/marketplace-actions.md.

Tier 2 (14 actions): actions/setup-dotnet, shivammathur/setup-php, aws-actions/configure-aws-credentials, google-github-actions/auth, azure/login, hashicorp/setup-terraform, codecov/codecov-action, softprops/action-gh-release, peter-evans/create-pull-request, JamesIves/github-pages-deploy-action, pnpm/action-setup, oven-sh/setup-bun, gradle/gradle-build-action, cypress-io/github-action.

Tier 3 (5 actions): tj-actions/changed-files, dorny/paths-filter (→ native rules:changes:), nick-invision/retry (→ native retry:), pre-commit/action, slackapi/slack-github-action.

Done when

• All 19 mappings land in action-mappings.ts with the same shape as feat: ActionMapping registry + Tier 1 mappings #87.
• Each has a unit test.
• Actions that map to a native GitLab keyword (dorny/paths-filter → rules:changes:, nick-invision/retry → retry:) emit the native form, not a script.

Depends on: #87.

Inspired by

gitlab-org/ci-cd/github-actions-to-gitlab-ci — references/marketplace-actions.md Tier 2 and Tier 3 sections.

[lex00]

Implemented in 77d6c6d on feat/migrate-github-to-gitlab. PR #99 is open (draft); CI green (check / test / validate / link-check all pass). Awaiting merge.