Completes the instructor-management half of #9: the studio admin can now
create instructor accounts and toggle each instructor's capabilities.
- InstructorController (manage_instructors): list instructors, create a
us_instructor WP user (emailing a set-password link), and a per-instructor
capability detail view.
- InstructorCapabilities: pure, unit-tested rules for which managed caps an
admin may assign and how a submitted form maps to assignments. Managed caps
are manage_offerings, manage_questions, view_own_payments, export_payments;
manage_availability and view_own_lessons are core to every instructor.
- A studio admin can never grant a capability it does not itself hold: only
held caps (checked via current_user_can, so an administrator's dynamic grant
counts) are offered, and on creation any managed cap the admin lacks is
denied on the new instructor so they never exceed their creator. The role
grants the managed caps by default; the page layers per-user overrides.
- AdminMenu: register the Instructors page in the people section.
- Tests for the capability logic; docs/features/user-roles.md updated.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
