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/content/find_all_content_files.phpt

@@ -0,0 +1,47 @@
+--TEST--
+findAllContentFiles: returns sorted content file paths, ignores non-content files and index.php
+--FILE--
+<?php
+require '/var/www/app/constants.php';
+require '/var/www/app/hooks.php';
+require '/var/www/app/content.php';
+
+$dir = sys_get_temp_dir() . '/phpt_content_' . getmypid();
+mkdir($dir);
+
+// Non-existent directory
+echo count(findAllContentFiles($dir . '/nope')) . "\n";
+
+// Empty directory
+echo count(findAllContentFiles($dir)) . "\n";
+
+// Content files
+file_put_contents("$dir/page.md", '# Hello');
+file_put_contents("$dir/extra.html", '<p>Hi</p>');
+file_put_contents("$dir/script.php", '<?php echo "hi";');
+
+// Non-content files are excluded
+file_put_contents("$dir/styles.css", 'body {}');
+file_put_contents("$dir/image.jpg", '');
+
+// index.php is always excluded
+file_put_contents("$dir/index.php", '<?php');
+
+$files = findAllContentFiles($dir);
+echo count($files) . "\n";
+
+// Results are full paths, sorted by filename (natural order)
+foreach ($files as $f) {
+    echo basename($f) . "\n";
+}
+
+array_map('unlink', glob("$dir/*"));
+rmdir($dir);
+?>
+--EXPECT--
+0
+0
+3
+extra.html
+page.md
+script.php