GitLab CI support

[structbinary]

Add GitLab CI (.gitlab-ci.yml) as a supported target platform for pipeline generation.

What needs to happen

Similar to the Jenkins issue — build a GitLab CI renderer that takes a CISpec and produces a valid .gitlab-ci.yml.

Scope

• New renderer module under ci_copilot/core/agents/ci_generation/gitlab_ci/
• .gitlab-ci.yml generation from CISpec
• Stage/job mapping, rules, artifacts, caching
• Support for GitLab-specific features (environments, includes, extends)
• Validation of generated YAML

Reference

• Existing pattern: ci_copilot/core/agents/ci_generation/github_ci/