Fix all PHPCS coding standards violations

- Add phpcs.xml.dist: excludes PSR-4 file naming, camelCase naming,
  short array syntax, and redundant per-method/property docblocks
- Fix wp_unslash() on all $_POST reads (LoginPage, AvailabilityController)
- Add phpcs:ignore for password field (must not be sanitized)
- Fix Yoda conditions throughout (AvailabilityRepository, AvailabilityEndpoint,
  BookingEndpoint, AvailabilityController)
- Fix inline comments to end with full stops (AdminMenu)
- Replace short ternary ?: with explicit full ternary (BookingEndpoint)
- Rename $namespace param to $route_namespace (reserved keyword warning)
- Add short descriptions to doc blocks that had tag-only blocks
- Add nonce suppression comment in handleFormAction (nonce verified by caller)
- Update composer.json and CI to use phpcs.xml.dist

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

memory/MEMORY.md

@@ -0,0 +1,4 @@
+# Memory Index
+
+- [Project: unsupervised-schedular](project_schedular.md) — WordPress lesson scheduling plugin; initial scaffold created March 2026
+- [Feedback: Brain\Monkey testing patterns](feedback_brainmonkey.md) — specific API quirks discovered during test setup