Add GitLab webhook support

[GodsBoy]

RepoKeeper currently only supports GitHub webhooks. Adding GitLab support would let users triage issues and review MRs on self-hosted GitLab instances.

Scope:

• Parse GitLab webhook payloads (issues, merge requests)
• Map GitLab events to existing triage/review pipeline
• Add GitLab API client for posting comments and labels

Resources:

• GitLab Webhooks docs
• Existing GitHub handler: src/webhook.ts

This is a great issue for contributors who want to make a significant impact.

[GodsBoy]

Thanks for the detailed feature request! GitLab webhook support is a natural expansion that would make RepoKeeper useful for self-hosted instances. The scope is well-defined and the resources you've linked should give contributors a solid foundation to work from. A maintainer will review this shortly and we can discuss implementation details with any interested contributors.