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/get_subdirectories.phpt

@@ -0,0 +1,39 @@
+--TEST--
+getSubdirectories: returns only subdirectory names, ignores files
+--FILE--
+<?php
+require '/var/www/app/helpers.php';
+
+$dir = sys_get_temp_dir() . '/phpt_subdirs_' . getmypid();
+mkdir($dir);
+
+// Empty directory
+$result = getSubdirectories($dir);
+echo (empty($result) ? 'empty' : 'not empty') . "\n";
+
+// Non-existent directory
+echo count(getSubdirectories($dir . '/nope')) . "\n";
+
+// Add subdirectories and a file
+mkdir("$dir/alpha");
+mkdir("$dir/beta");
+touch("$dir/file.txt");
+
+$result = array_values(getSubdirectories($dir));
+sort($result);
+echo implode("\n", $result) . "\n";
+
+// Files are not included
+echo in_array('file.txt', $result) ? "file included\n" : "file excluded\n";
+
+unlink("$dir/file.txt");
+rmdir("$dir/alpha");
+rmdir("$dir/beta");
+rmdir($dir);
+?>
+--EXPECT--
+empty
+0
+alpha
+beta
+file excluded