Update framework testing infrastructure and standards

- Add phpt test runner and suite for app functions
- Introduce testing workflow to AGENT.md
- Add tests for cache, content, context, helpers, hooks, plugins,
  rendering
- Mount tests directory in dev container

devel/tests/run.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+# Run phpt tests inside the container.
+# Usage: ./run.sh [filter]   — filter matches against test filename substrings
+# Example: ./run.sh extract_raw_date
+
+CONTAINER=${CONTAINER:-folderweb-default}
+
+podman exec "$CONTAINER" php /var/www/tests/run.php "$@"