Add README and student-administration feature spec

- README.md: plugin overview, package-by-domain architecture, feature/status
  table linking each docs/features spec (implemented vs planned), shortcodes,
  REST namespace, roles, install via composer build, dev commands, and CI.
- docs/features/student-administration.md: spec for the planned studio-admin
  Students list + per-student detail (upcoming/past lessons, group enrolments);
  read-only; manage_students; no new tables. Tracked as #22.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/features/student-administration.md

@@ -0,0 +1,46 @@
+# Feature: Student Administration
+
+## Overview
+A read-only studio-admin area to browse students and drill into one student's
+history and upcoming activity — lessons and group-class enrolments — without
+digging through individual records. (Status: planned — issue #22.)
+
+## Data Model
+No new tables. The views are composed from existing data:
+- Students are WordPress users with the `us_student` role (`get_users`, `get_userdata`).
+- Lessons come from `{prefix}us_lessons` (with `{prefix}us_availability` for slot times).
+- Group-class enrolments come from `{prefix}us_group_enrollments`.
+
+## Admin Interface
+**Students** in wp-admin (`manage_students`, studio admin only):
+
+- **List** — every `us_student` user: display name, email, registered date, and
+  quick counts (upcoming lessons, active group enrolments). Each row links to the
+  detail view.
+- **Detail** (`?student_id=`):
+  - **Account** — display name, email, registered date.
+  - **Upcoming lessons** and **Past lessons** — split by the linked availability
+    slot's `start_dt`; each shows date/time, offering, instructor, and status.
+  - **Group-class enrolments** — active/past, with offering title and status.
+  - *(Later)* policy-acceptance history, intake answers, and payment history once
+    Payments lands.
+
+Read-only in this iteration; cancel/edit actions are a possible follow-up.
+
+## Capabilities
+- `manage_students` — studio admin (administrators inherit it via the
+  `user_has_cap` filter). No new capabilities or tables.
+
+## Implementation
+- Admin controller: `Unsupervised\Schedular\Auth\StudentController` (list + detail)
+- Templates: `templates/admin/students.php`, `templates/admin/student-detail.php`
+- Reuses `Booking\BookingRepository::findByStudent`,
+  `Availability\AvailabilityRepository::findById`,
+  `Offering\OfferingRepository::findById`,
+  `GroupClass\EnrollmentRepository::findByStudent`
+- The upcoming/past split is extracted into a small pure helper so it is
+  unit-testable (the controller itself follows the repo convention of not being
+  unit-tested).
+
+## Tests
+- `tests/Unit/Auth/StudentScheduleTest.php` (the pure upcoming/past split helper)