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_raw_date_from_folder.phpt

@@ -0,0 +1,20 @@
+--TEST--
+extractRawDateFromFolder: extracts YYYY-MM-DD from date-prefixed folder names
+--FILE--
+<?php
+require '/var/www/app/helpers.php';
+
+echo extractRawDateFromFolder('2024-01-15-my-post') . "\n";
+echo extractRawDateFromFolder('2024-12-31-year-end') . "\n";
+echo (extractRawDateFromFolder('no-date-here') ?? 'null') . "\n";
+echo (extractRawDateFromFolder('') ?? 'null') . "\n";
+echo (extractRawDateFromFolder('2024-1-1-bad-format') ?? 'null') . "\n";
+echo (extractRawDateFromFolder('20240115-no-separators') ?? 'null') . "\n";
+?>
+--EXPECT--
+2024-01-15
+2024-12-31
+null
+null
+null
+null