Restrict Skills WG admin access to WG leads

[sambhav]

The full Skills WG currently has admin access to ext-skills. This separates repository administration from regular WG contributions.

• Create the GitHub-only skills-over-mcp-wg-leads team under skills-over-mcp-wg, containing exactly sambhav, olaservo (Ola Hungerford), and pja-ant (Peter Alexander).
• Grant the leads team admin and the full WG write (push) on ext-skills, allowing WG members to create branches and collaborate.
• Retain the explicit core-maintainers admin grant, as requested in review.
• Reduce the repository's moderator grant from maintain to write so overlapping WG membership does not retain maintain access.

Organization owners and the existing org-wide admin roles retain their access. Within the Skills WG, admin is granted only to the three leads.

Follow-up after the leads team exists: update ext-skills/.github/CODEOWNERS to the leads team (it currently references the old skills-over-mcp-ig team) and verify that branch protection requires code-owner approval. Write access alone does not enforce that review requirement.

Validation: all 33 existing configuration tests and reference validation passed again after the review change, using Node 24's TypeScript support in a temporary copy with explicit import extensions. No infrastructure deployment was performed.