Add skills installer, GitLab bundle extension, and capability catalog

[zloeber]

Summary

• Add secretzero skills for installing bundled agent skills into Cursor, Claude Code, Hermes, OpenClaw, and other targets (list, show, install with --scope / --target / --dry-run).
• Extend the GitLab provider bundle with hardened CI/CD variable sync (gitlab_group_variable registration, shared variable helpers, project: auto resolution) and a gitlab_project_token generator for scoped project access tokens.
• Add secretzero catalog --format json as a machine-complete registry of generator kinds, target kinds, and provider bundles backed by BundleRegistry; wire secret-types to the same source.
• Update secretzero-author skills and internal Cursor skill copies to prefer the catalog for agent discovery, fix stale CLI references, and document GitLab target/generator options.

Test plan

• pytest tests/test_bundle_catalog.py tests/test_gitlab_*.py tests/test_cli_skills.py tests/test_skills_installer.py
• ./scripts/agent.pre-commit.sh --mode fast --quiet
• secretzero catalog --format json — verify gitlab_project_token and gitlab_group_variable appear
• secretzero skills list / secretzero skills install --dry-run --scope project
• Dry-run sync against examples/gitlab-project-token.yml when GITLAB_TOKEN is available