Document GitLab CI report integration

[amic25]

Goal

Add a tested GitLab CI example under docs/integrations/.

Scope

• Show installation, scanning, threshold behavior, and artifact retention.
• Use a machine-readable AgentGuard report.
• State limitations accurately; do not claim native GitLab behavior AgentGuard does not provide.
• Link the guide from the README.

Acceptance criteria

• YAML is syntactically valid.
• Commands match the current CLI.
• The guide explains exit codes and report artifacts.

[deekshithasajja-maker]

Hi!👋 @amic25 I'd like to work on this issue. Could you please assign it to me? Thanks!