GitLab plugin support

[ewega]

Summary

Add GitLab as a supported DevOps tool in the CLI. This is the first non-GitHub plugin, validating the multi-tool architecture.

Changes

• Add new ConnectionDef for GitLab in connectionRegistry
• GitLab-specific fields: endpoint (default gitlab.com), private token
• GitLab scope handling: groups/projects instead of orgs/repos
• GitLab token resolution chain
• Set Available: true on the GitLab entry

Acceptance Criteria

• gh devlake configure connection --plugin gitlab creates a GitLab connection
• GitLab appears in interactive plugin picker
• GitLab scopes work with groups/projects
• Connection test works against GitLab API