Limit GitLab crawling to configured host org

[AgentGoose32]

Refs #483

Summary

• Uses the existing hosts.org configuration to scope GitLab crawling to a configured public group instead of walking the entire forge.
• Applies the group scope to both synchronous and asynchronous crawl paths.
• Adds regression coverage to verify scoped hosts call the GitLab group projects endpoint and do not fall back to forge-wide project crawling.

Validation

• ruby -c app/models/hosts/gitlab.rb
• ruby -c test/models/hosts/gitlab_test.rb
• git diff --check

bundle exec ruby -Itest test/models/hosts/gitlab_test.rb remains blocked locally because this checkout requires Bundler 4.0.10 from Gemfile.lock, which is not available in the system Ruby environment.