Add gitlab-backport agent skill

What does this MR do and why?

Adds the gitlab-backport agent skill to .claude/skills/, so this repo becomes its source of truth, like postgresai. The skill's contributing section now points here instead of gitlab-org/ai/skills. The skill directory is also allowlisted in the AI harness doctor.

Merge request checklist

• Content copied from gitlab-org/ai/skills; only the contributing section changed.
• Added with git add --force, per .claude/skills/README.md.

Agent context

• Source: gitlab-org/ai/skills at 69fea3b0, skills/gitlab-backport/ (SKILL.md, references/).
• Skill version bumped to 1.0.1 for the changed contributing section.
• Added the directory to allowed_committed_files in tooling/ai_harness/config.yml and a matching doctor spec, as in the postgresai change.
• Ran the ai_harness specs and doctor: green. Markdownlint was not run locally.
• No project issue; labels and milestone follow precedent MR !260040 (merged).