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/build_cache_key.phpt

@@ -0,0 +1,49 @@
+--TEST--
+buildCacheKey: returns md5 string, varies by path and langPrefix
+--FILE--
+<?php
+require '/var/www/app/cache.php';
+
+$dir = sys_get_temp_dir() . '/phpt_cachekey_' . getmypid();
+mkdir($dir);
+$file = "$dir/page.md";
+file_put_contents($file, '# Hello');
+
+// Returns a 32-char hex string
+$key = buildCacheKey($file);
+echo (bool)preg_match('/^[0-9a-f]{32}$/', $key) ? 'valid md5' : 'invalid';
+echo "\n";
+
+// Same inputs produce same key
+echo ($key === buildCacheKey($file)) ? 'stable' : 'unstable';
+echo "\n";
+
+// Different langPrefix produces different key
+$keyFr = buildCacheKey($file, '/fr');
+echo ($key !== $keyFr) ? 'prefix differs' : 'prefix same';
+echo "\n";
+
+// Different file path produces different key
+$file2 = "$dir/other.md";
+file_put_contents($file2, '# Other');
+$key2 = buildCacheKey($file2);
+echo ($key !== $key2) ? 'path differs' : 'path same';
+echo "\n";
+
+// Adding metadata.ini changes the key
+file_put_contents("$dir/metadata.ini", "title = Test\n");
+$keyWithMeta = buildCacheKey($file);
+echo ($key !== $keyWithMeta) ? 'meta invalidates' : 'meta ignored';
+echo "\n";
+
+unlink($file);
+unlink($file2);
+unlink("$dir/metadata.ini");
+rmdir($dir);
+?>
+--EXPECT--
+valid md5
+stable
+prefix differs
+path differs
+meta invalidates