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/helpers/find_cover_image.phpt

@@ -0,0 +1,33 @@
+--TEST--
+findCoverImage: finds cover.{ext} in directory, prefers first matching extension
+--FILE--
+<?php
+require '/var/www/app/constants.php';
+require '/var/www/app/helpers.php';
+
+$dir = sys_get_temp_dir() . '/phpt_cover_' . getmypid();
+mkdir($dir);
+
+// No cover image
+echo (findCoverImage($dir) ?? 'null') . "\n";
+
+// cover.jpg
+touch("$dir/cover.jpg");
+echo findCoverImage($dir) . "\n";
+
+// cover.webp added — jpg should still win (it's first in COVER_IMAGE_EXTENSIONS)
+touch("$dir/cover.webp");
+echo findCoverImage($dir) . "\n";
+
+// Remove jpg — webp should now be found
+unlink("$dir/cover.jpg");
+echo findCoverImage($dir) . "\n";
+
+array_map('unlink', glob("$dir/*"));
+rmdir($dir);
+?>
+--EXPECT--
+null
+cover.jpg
+cover.jpg
+cover.webp