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/find_page_js.phpt

@@ -0,0 +1,36 @@
+--TEST--
+findPageJs: returns URL and hash for script.js, null when absent
+--FILE--
+<?php
+require '/var/www/app/helpers.php';
+
+$contentDir = sys_get_temp_dir() . '/phpt_js_' . getmypid();
+$pageDir = "$contentDir/section/page";
+mkdir($pageDir, 0777, true);
+
+// No script.js
+echo (findPageJs($pageDir, $contentDir) ?? 'null') . "\n";
+
+// Add script.js
+file_put_contents("$pageDir/script.js", 'console.log("hi");');
+$result = findPageJs($pageDir, $contentDir);
+echo $result['url'] . "\n";
+echo ($result['hash'] === md5_file("$pageDir/script.js") ? 'hash ok' : 'hash mismatch') . "\n";
+
+// Root-level page (no subpath)
+$rootJs = "$contentDir/script.js";
+file_put_contents($rootJs, '');
+$result = findPageJs($contentDir, $contentDir);
+echo $result['url'] . "\n";
+
+unlink("$pageDir/script.js");
+unlink($rootJs);
+rmdir($pageDir);
+rmdir("$contentDir/section");
+rmdir($contentDir);
+?>
+--EXPECT--
+null
+/section/page/script.js
+hash ok
+/script.js