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/load_metadata.phpt

@@ -0,0 +1,44 @@
+--TEST--
+loadMetadata: parses metadata.ini, returns null when absent
+--FILE--
+<?php
+require '/var/www/app/hooks.php';
+require '/var/www/app/content.php';
+
+$dir = sys_get_temp_dir() . '/phpt_meta_' . getmypid();
+mkdir($dir);
+
+// No metadata file
+echo (loadMetadata($dir) ?? 'null') . "\n";
+
+// Basic key-value metadata
+file_put_contents("$dir/metadata.ini", "title = My Page\nsummary = A summary\nmenu = true\n");
+$meta = loadMetadata($dir);
+echo $meta['title'] . "\n";
+echo $meta['summary'] . "\n";
+echo $meta['menu'] . "\n";
+
+// _raw is always present
+echo isset($meta['_raw']) ? 'raw present' : 'raw missing';
+echo "\n";
+
+// Sections are in _raw but not in base metadata
+file_put_contents("$dir/metadata.ini", "[en]\ntitle = English Title\n\n[fr]\ntitle = French Title\n");
+$meta = loadMetadata($dir);
+echo isset($meta['en']) ? 'section leaked' : 'section contained';
+echo "\n";
+echo $meta['_raw']['en']['title'] . "\n";
+echo $meta['_raw']['fr']['title'] . "\n";
+
+array_map('unlink', glob("$dir/*"));
+rmdir($dir);
+?>
+--EXPECT--
+null
+My Page
+A summary
+1
+raw present
+section contained
+English Title
+French Title