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/content/resolve_slug_to_folder.phpt

@@ -0,0 +1,48 @@
+--TEST--
+resolveSlugToFolder: resolves URL slug to actual folder name, supports metadata slug override
+--FILE--
+<?php
+require '/var/www/app/hooks.php';
+require '/var/www/app/content.php';
+
+$parent = sys_get_temp_dir() . '/phpt_slug_' . getmypid();
+mkdir($parent);
+
+// Non-existent parent directory
+echo (resolveSlugToFolder($parent . '/nope', 'anything') ?? 'null') . "\n";
+
+// Folder name matches slug directly
+mkdir("$parent/my-post");
+echo resolveSlugToFolder($parent, 'my-post') . "\n";
+
+// Slug not found
+echo (resolveSlugToFolder($parent, 'missing') ?? 'null') . "\n";
+
+// Metadata slug overrides folder name
+mkdir("$parent/2024-01-15-article");
+file_put_contents("$parent/2024-01-15-article/metadata.ini", "slug = article\n");
+echo resolveSlugToFolder($parent, 'article') . "\n";
+
+// Original folder name still works when no slug conflicts
+echo resolveSlugToFolder($parent, '2024-01-15-article') . "\n";
+
+// Metadata slug takes precedence over folder with that name
+mkdir("$parent/other");
+file_put_contents("$parent/other/metadata.ini", "slug = my-post\n");
+// 'my-post' folder exists AND 'other' has slug=my-post — folder name wins (first match)
+echo resolveSlugToFolder($parent, 'my-post') . "\n";
+
+array_map('unlink', glob("$parent/2024-01-15-article/*"));
+rmdir("$parent/2024-01-15-article");
+array_map('unlink', glob("$parent/other/*"));
+rmdir("$parent/other");
+rmdir("$parent/my-post");
+rmdir($parent);
+?>
+--EXPECT--
+null
+my-post
+null
+2024-01-15-article
+2024-01-15-article
+my-post