Add Gerrit host support

[AgentGoose32]

Refs #606

Summary

• Adds a Gerrit host adapter using the Gerrit REST API project endpoints.
• Maps Gerrit project metadata into repository fields and exposes Gitiles HTML/raw/archive URLs.
• Adds project listing/crawl support, host version lookup, seed entries from the issue, and model coverage.

Validation

• ruby -c app/models/hosts/gerrit.rb
• ruby -c test/models/gerrit_test.rb
• ruby -c db/seeds.rb
• git diff --check

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