Establish maintainer governance and team-based CODEOWNERS

[breadoncee]

Context

chalk-skills is currently using individual CODEOWNERS as a temporary setup because org/team governance is not finalized.

Goal

Move to team-based maintainership and enforce review policy via branch protection.

Tasks

• Create GitHub org team (e.g. maintainers)
• Grant the team write/maintain access to this repository
• Update .github/CODEOWNERS from individual owners to team owner (@<org>/maintainers)
• Enable branch protection on main:
  • Require pull requests
  • Require status check: Validate Skills
  • Require CODEOWNERS review
• Update CONTRIBUTING.md maintainer section after team migration

Notes

• Keep current individual CODEOWNERS until org/team setup is complete.
• Close this issue only after team-based CODEOWNERS is active and branch protections are enforced.