Initial plugin scaffold: lesson scheduling WordPress plugin

- Custom DB tables for availability slots and lesson bookings
- Instructor (wp-admin) and student (front-end) roles with custom capabilities
- REST API under us-scheduler/v1 for availability CRUD and booking
- [us_booking] and [us_student_login] shortcodes for student front end
- PHPUnit + Brain\Monkey unit test suite (29 tests)
- Gitea Actions CI: lint, PHPStan, tests on PHP 8.1/8.2/8.3, no-debug check
- Feature docs under docs/features/

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

phpstan.neon

@@ -0,0 +1,11 @@
+includes:
+    - vendor/szepeviktor/phpstan-wordpress/extension.neon
+
+parameters:
+    level: 6
+    paths:
+        - src
+    bootstrapFiles:
+        - tests/bootstrap.php
+    ignoreErrors:
+        - '#Unsafe usage of new static\(\)#'