Function export_gitlab_ci 

pub fn export_gitlab_ci(
    session: &Session,
    params: Option<&[Parameter]>,
) -> String

Export a session as a GitLab CI configuration YAML.

Generates a .gitlab-ci.yml with:

• Metadata header comments (session name, date, command count)
• Single build stage and job
• ubuntu:22.04 Docker image
• variables: block when parameters are provided
• cd commands in the script array when cwd changes
• Commands passed through escape_yaml() for safe YAML output
• Annotations for failed commands (non-zero exit codes)

When params is Some, a variables: block is added at job level and commands use $VAR references.

Empty sessions produce a valid config with a placeholder script entry (GitLab CI requires at least one script entry).