feat(proto): add gkg-proto Ruby gem build pipeline

Adds tooling to build and publish a gitlab-gkg-proto RubyGem from crates/gkg-server/proto/gkg.proto, following the Gitaly pattern. Rails can then consume proto files via Bundler instead of committing generated files.

Gem published

The gem name gitlab-gkg-proto has been reserved on RubyGems.org (v0.0.1 placeholder). Naming follows the gitlab- prefix convention.

Owners:

RubyGems handle	Name
arivera-gitlab	Angelo Rivera
gitlab_rubygems	GitLab (org account)
marinjankovski	Marin Jankovski
rymai	Rémy Coutable
stanhu	Stan Hu

All co-owners besides arivera-gitlab are pending email confirmation.

Changes

• tools/protogem/build-proto-gem — build script that runs grpc_tools_ruby_protoc and packages the output as a gem
• tools/protogem/Gemfile + Gemfile.lock — grpc-tools build dependency
• VERSION — initial 0.1.0 (fallback for local dev; tagged pipelines use $CI_COMMIT_TAG)
• mise.toml — proto:build-gem and proto:publish-gem tasks
• .gitlab-ci.yml — build-proto-gem and publish-proto-gem CI jobs

How it works

Locally:

mise run proto:build-gem    # → _build/gitlab-gkg-proto-0.1.0.gem

In CI:

• build-proto-gem runs on MR proto changes, version tags, or manual trigger
• publish-proto-gem runs on version tags only (same trigger as Docker releases via semantic-release)
• Version is derived from $CI_COMMIT_TAG, matching the existing release pipeline
• Uses GEM_HOST_API_KEY CI variable for RubyGems auth (needs to be set in project settings)

Downstream (separate MR in gitlab-org/gitlab)

gem 'gitlab-gkg-proto', '~> 0.1.0'

Closes #161 (closed)