Add gitlab-org-contribution skill

New Skill: gitlab-org-contribution

Description

Guides a community contributor with no role in the project, or an agent working for one, through merge requests and issues in GitLab's own projects on gitlab.com: claiming an issue through contributors.gitlab.com, the community forks, commits and descriptions that pass Danger, @gitlab-bot ready, answering reviewers and GitLab Duo, pipelines a contributor cannot start or retry, and why community MRs get closed. It is not for GitLab team members or anyone with at least the Developer role in the project, GitHub, or a self-managed instance.

This is the local copy suggested in #19 (closed), maintained here. skills/gitlab-org-contribution/ is release v0.7.1 of jmrp/gitlab-org-contribution-skill without its optional activity monitor, so it is SKILL.md and references/ only, all Markdown, and its Contributing Improvements section points here. The rest of the diff is what bin/distribute-skill and lefthook run auto-fix generate. Before each later release MR, I take any change made here back to my repository first.

The project rules sit one level further down, in references/projects/ (one file per project for 36 projects, and other-projects.md for 25 with too little material for a file of their own), reached from SKILL.md through references/projects/INDEX.md, because a contribution needs only its own project's file and SKILL.md says how to find it from the project's path. Every rule cites public evidence and says whether it is enforced, documented or observed, and the skill names no individuals. This MR comes from a personal fork, since gitlab-community has no fork of this project.

Closes #19 (closed)

Checklist

• Created skills/gitlab-org-contribution/SKILL.md with required frontmatter (name, description, license, version)
• Ran bin/distribute-skill to define distribution rules for your skill
• Ran lefthook run auto-fix to auto-update docs and manifests
• All CI jobs pass
• Skill content follows existing conventions (see skills/glab/SKILL.md as reference)

Checks run before pushing

• The verify and lint jobs of .gitlab-ci.yml, run as it writes them, and bin/ci-checks, on main at c12add8c: all pass. They ran in python:3.12-alpine3.20 with CI's packages, PyYAML 6.0.2 and ruff 0.17.2, and test-skill-hooks-plugin also in its own image, node:22-alpine. Content quality reports no broken link and no warning for this skill, and SKILL.md is 93 lines.
• The skill directory is the v0.7.1 tag's skills/gitlab-org-contribution/ without scripts/, tests/, references/monitor.md and the three places in SKILL.md that describe the monitor, and with this repository in Contributing Improvements.