feat(ycst-org-uk): add wp-mu-plugins private repo

[robinbowes]

Adds data/ycst-org-uk/wp-mu-plugins.yaml.

The repo holds the must-use WordPress plugins for www.ycst.org.uk and their tooling. The QR
short-link plugin behind the membership sign-up codes currently exists only on the production
and clone servers, as do four others including the CiviCRM/Matomo autoloader guard.

Private, not internal. Internal visibility requires the org to belong to a GitHub Enterprise
account; ycst-org-uk is on the free plan, so the API would reject it at apply time. Private
matches board-docs and website-testing.

auto_init is omitted (module default false) so the repository is created empty and the
existing local history can be pushed into it without a merge.

Scoped plan against module.org_ycst_org_uk:

Plan: 4 to add, 0 to change, 0 to destroy.

+ github_repository                              wp-mu-plugins
+ github_repository_collaborators                wp-mu-plugins  (admins, admin)
+ github_repository_dependabot_security_updates  wp-mu-plugins
+ github_repository_vulnerability_alerts         wp-mu-plugins

Everything belonging to board-docs and website-testing plans as no-op. The admins slug
resolves to team 18962366, the same team those two already grant to.

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