Add a repo-root `skills/` catalog so `npx skills add recodeee/gitguardex`
shows the Guardex skills before install, while keeping
`gx install-agent-skills` as the Codex/Claude user-home path.
README, the merge-skill runbook, and metadata coverage now all track the
new root skill catalog.

Constraint: `npx skills` discovers repo-root `skills/**/SKILL.md`; existing `gx install-agent-skills` still needs to keep the Codex/Claude user-home flow
Rejected: Replace `gx install-agent-skills` with `npx skills` only | would drop the Claude user-home command path
Confidence: high
Scope-risk: narrow
Directive: Keep `skills/**/SKILL.md` byte-aligned with `templates/codex/skills/**/SKILL.md` when editing Guardex skills
Tested: node --test test/metadata.test.js; git diff --check
Not-tested: live `npx skills add recodeee/gitguardex` network install