Add GitLab CI adoption note

[baskduf]

Goal

Document how maintainers should think about harness adoption when a target repository uses GitLab CI instead of GitHub Actions.

Scope

• Add a concise note or example under docs/.
• Explain that CI wiring should fit the target repository's existing provider.
• Show which generic checks can be run in GitLab CI without copying the GitHub Actions workflow blindly.
• Link from README or validation docs if useful.

Acceptance Criteria

• The note preserves prompt-first adoption and target-source-of-truth guidance.
• It does not introduce a mandatory .gitlab-ci.yml template.
• python scripts/check_docs_drift.py passes.
• Full local validation still passes.

References

• ROADMAP.md
• CONTRIBUTING.md
• docs/adoption-workflow.md