Add a GitLab CI template alongside the GitHub Action

[amarjaleelbanbhan]

VeriPatch ships a GitHub composite Action (action.yml) but no equivalent for GitLab CI. A
.gitlab-ci.yml snippet (or a small example under examples/) that runs veripatch scan
(and optionally verify, gated behind a Docker-in-Docker runner) would let GitLab users adopt
this without hand-rolling it.

Scope:

• examples/gitlab-ci.yml showing scan (and optionally verify) as a job
• A short section in docs/CLI.md or a new docs/GITLAB_CI.md explaining the DinD requirement for verify
• No changes to src/ needed — this is docs/examples only, good first-issue scope

See .github/workflows/ci.yml and action.yml for what the equivalent GitHub-side job does.