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_title.phpt

@@ -0,0 +1,49 @@
+--TEST--
+extractTitle: extracts title from # heading in .md or <h1> in .html
+--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';
+
+$dir = sys_get_temp_dir() . '/phpt_title_' . getmypid();
+mkdir($dir);
+
+// No content files
+echo (extractTitle($dir) ?? 'null') . "\n";
+
+// Markdown with # heading
+file_put_contents("$dir/index.md", "# My Great Post\n\nSome body text.");
+echo extractTitle($dir) . "\n";
+unlink("$dir/index.md");
+
+// Markdown heading with extra whitespace
+file_put_contents("$dir/index.md", "#  Spaced Title  \n\nBody.");
+echo extractTitle($dir) . "\n";
+unlink("$dir/index.md");
+
+// HTML with <h1>
+file_put_contents("$dir/index.html", "<h1>HTML Title</h1><p>Body</p>");
+echo extractTitle($dir) . "\n";
+unlink("$dir/index.html");
+
+// HTML with attributes on h1
+file_put_contents("$dir/index.html", '<h1 class="main">Styled Title</h1>');
+echo extractTitle($dir) . "\n";
+unlink("$dir/index.html");
+
+// Markdown without heading
+file_put_contents("$dir/index.md", "Just a paragraph, no heading.");
+echo (extractTitle($dir) ?? 'null') . "\n";
+unlink("$dir/index.md");
+
+rmdir($dir);
+?>
+--EXPECT--
+null
+My Great Post
+Spaced Title
+HTML Title
+Styled Title
+null