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/cache/get_set_cached_markdown.phpt

@@ -0,0 +1,36 @@
+--TEST--
+getCachedMarkdown / setCachedMarkdown: store and retrieve rendered HTML by file key
+--FILE--
+<?php
+require '/var/www/app/cache.php';
+
+$dir = sys_get_temp_dir() . '/phpt_mdcache_' . getmypid();
+mkdir($dir);
+$file = "$dir/post.md";
+file_put_contents($file, '# Hello');
+
+// Nothing cached yet
+echo (getCachedMarkdown($file) ?? 'null') . "\n";
+
+// Store and retrieve
+setCachedMarkdown($file, '<h1>Hello</h1>');
+echo getCachedMarkdown($file) . "\n";
+
+// langPrefix produces a separate cache entry
+echo (getCachedMarkdown($file, '/fr') ?? 'null') . "\n";
+
+setCachedMarkdown($file, '<h1>Bonjour</h1>', '/fr');
+echo getCachedMarkdown($file, '/fr') . "\n";
+
+// Default key is unaffected
+echo getCachedMarkdown($file) . "\n";
+
+unlink($file);
+rmdir($dir);
+?>
+--EXPECT--
+null
+<h1>Hello</h1>
+null
+<h1>Bonjour</h1>
+<h1>Hello</h1>