v1.13.0 — GitLab Pages platform module

Third step toward "support as many framework/platform combos as possible, zero new accounts or auth" (NEXT_ACTIONS.md #20-#24).

#22 — GitLab Pages platform module
New src/platforms/gitlab-pages.js — static-only warning fixer, same shape as github-pages.js (no server-side Accept-header branching possible on either static host).

detect-stack.js gained hasGitlabPagesJob(), detecting .gitlab-ci.yml + a top-level pages: job as its own platform signal (no YAML parse dependency, matching the existing config-file-existence style used for every other platform signal). Applied to both the npm-ecosystem branch (astro/astro-starlight + gitlab-pages is now a supported enhance pairing) and the Jekyll branch (previously hardcoded to github-pages, now gitlab-pages or github-pages depending on the CI config present).

loop.js's runScanLocal guard was generalized from a per-platform allowlist to excluding Jekyll by framework (its build is Ruby/bundler, not npm run build, regardless of which Pages host it targets) — so gitlab-pages, and any future platform detect-stack.js learns, rides scan-local automatically with no allowlist to keep extending.

Docs (README.md, docs/install.md, docs/why.md, docs/architecture.md) updated for this and the previous release's Netlify + generic static-server changes.

Verified: detectStack() across 5 synthetic repos (including 2 regression checks confirming plain Jekyll and plain Astro behavior is unchanged), idempotent enhance(), runScanLocal correctly rejects Jekyll regardless of platform, npm run verify-loop green.

Full detail: WORKLOG.md v1.13.0.