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_css.phpt

@@ -0,0 +1,42 @@
+--TEST--
+findPageCss: returns URL and hash for styles.css, null when absent
+--FILE--
+<?php
+require '/var/www/app/helpers.php';
+
+$contentDir = sys_get_temp_dir() . '/phpt_css_' . getmypid();
+$pageDir = "$contentDir/blog/post";
+mkdir($pageDir, 0777, true);
+
+// No styles.css
+echo (findPageCss($pageDir, $contentDir) ?? 'null') . "\n";
+
+// Add styles.css
+file_put_contents("$pageDir/styles.css", 'body { color: red; }');
+$result = findPageCss($pageDir, $contentDir);
+echo $result['url'] . "\n";
+echo ($result['hash'] === md5_file("$pageDir/styles.css") ? 'hash ok' : 'hash mismatch') . "\n";
+
+// Root-level page (no subpath)
+$rootCss = "$contentDir/styles.css";
+file_put_contents($rootCss, 'body {}');
+$result = findPageCss($contentDir, $contentDir);
+echo $result['url'] . "\n";
+
+// Directory named styles.css is ignored
+unlink("$pageDir/styles.css");
+mkdir("$pageDir/styles.css");
+echo (findPageCss($pageDir, $contentDir) ?? 'null') . "\n";
+
+rmdir("$pageDir/styles.css");
+unlink($rootCss);
+rmdir($pageDir);
+rmdir("$contentDir/blog");
+rmdir($contentDir);
+?>
+--EXPECT--
+null
+/blog/post/styles.css
+hash ok
+/styles.css
+null