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/extract_meta_description.phpt

@@ -0,0 +1,45 @@
+--TEST--
+extractMetaDescription: returns description from metadata fields or first content paragraph
+--FILE--
+<?php
+require '/var/www/app/constants.php';
+require '/var/www/app/hooks.php';
+require '/var/www/app/content.php';
+require '/var/www/app/helpers.php';
+
+$dir = sys_get_temp_dir() . '/phpt_metadesc_' . getmypid();
+mkdir($dir);
+
+// search_description takes priority
+echo extractMetaDescription($dir, ['search_description' => 'SEO description', 'summary' => 'Summary']) . "\n";
+
+// summary is fallback when no search_description
+echo extractMetaDescription($dir, ['summary' => 'Page summary']) . "\n";
+
+// null metadata, no content files
+echo (extractMetaDescription($dir, null) ?? 'null') . "\n";
+
+// First paragraph from markdown (skips headings and short lines)
+file_put_contents("$dir/index.md", "# Title\n\nThis is the opening paragraph of the post.\n\nSecond paragraph.");
+echo extractMetaDescription($dir, null) . "\n";
+unlink("$dir/index.md");
+
+// Heading-only markdown with no usable paragraph
+file_put_contents("$dir/index.md", "# Just a Heading\n\nShort.\n");
+echo (extractMetaDescription($dir, null) ?? 'null') . "\n";
+unlink("$dir/index.md");
+
+// First <p> from HTML
+file_put_contents("$dir/index.html", "<h1>Title</h1><p>HTML paragraph text here.</p>");
+echo extractMetaDescription($dir, null) . "\n";
+unlink("$dir/index.html");
+
+rmdir($dir);
+?>
+--EXPECT--
+SEO description
+Page summary
+null
+This is the opening paragraph of the post.
+null
+HTML paragraph text here.