feat(skills): add Atheris fuzz harness for GitLab skill

[WilliamBerryiii]

Summary

The GitLab skill (.github/skills/gitlab/gitlab/) has a tests/ directory but is missing the required tests/fuzz_harness.py file. After PR #1102 merges, npm run validate:skills will fail with an error-level check for any Python skill that has tests but no fuzz harness.

Required Changes

1. Add tests/fuzz_harness.py — Polyglot Atheris fuzz harness following the pattern established in .github/skills/experimental/powerpoint/tests/fuzz_harness.py.
2. Add fuzz dependency group in pyproject.toml with atheris>=3.0 (kept separate from dev — no macOS wheels).
3. Add fuzz_harness.py to python_files in [tool.pytest.ini_options] to enable pytest discovery.

Context

• Validation rule: Validate-SkillStructure.ps1 → Test-PythonSkillConfig enforces tests/fuzz_harness.py at error level for Python skills with a tests/ directory.
• Convention docs: docs/security/fuzzing.md, docs/contributing/skills.md
• Reference implementation: .github/skills/experimental/powerpoint/tests/fuzz_harness.py
• Introduced by: feat(skills): add Atheris fuzz harness with CI workflow integration #1102

Affected PR

• feat(integrations): add Jira backlog workflows and Jira/GitLab skills #978 — feat(integrations): add Jira backlog workflows and Jira/GitLab skills