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/plugins/plugin_manager.phpt

@@ -0,0 +1,59 @@
+--TEST--
+PluginManager: tracks loaded plugins, deduplicate loads, respects scope
+--FILE--
+<?php
+require '/var/www/app/hooks.php';
+require '/var/www/app/context.php';
+require '/var/www/app/plugins.php';
+
+$pm = new PluginManager();
+
+// Initially empty
+echo count($pm->getLoadedPlugins()) . "\n";
+echo count($pm->getGlobalPlugins()) . "\n";
+echo ($pm->isLoaded('languages') ? 'yes' : 'no') . "\n";
+echo ($pm->getPluginInfo('languages') ?? 'null') . "\n";
+
+// loadPagePlugins with null — no-op
+$pm->loadPagePlugins(null);
+echo count($pm->getLoadedPlugins()) . "\n";
+
+// loadPagePlugins with no plugins key — no-op
+$pm->loadPagePlugins(['title' => 'My Page']);
+echo count($pm->getLoadedPlugins()) . "\n";
+
+// loadGlobalPlugins with no enabled key — no-op
+$pm->loadGlobalPlugins(['other' => 'stuff']);
+echo count($pm->getLoadedPlugins()) . "\n";
+
+// Load the built-in languages plugin via global config
+$pm->loadGlobalPlugins(['plugins' => ['enabled' => 'languages']]);
+echo ($pm->isLoaded('languages') ? 'yes' : 'no') . "\n";
+echo count($pm->getGlobalPlugins()) . "\n";
+echo in_array('languages', $pm->getGlobalPlugins()) ? 'in global' : 'not in global';
+echo "\n";
+
+// Loading again is a no-op (deduplication)
+$pm->loadGlobalPlugins(['plugins' => ['enabled' => 'languages']]);
+echo count($pm->getLoadedPlugins()) . "\n";
+
+// getPluginInfo returns path and scope
+$info = $pm->getPluginInfo('languages');
+echo ($info['scope'] === 'global' ? 'scope ok' : 'scope wrong') . "\n";
+echo (str_ends_with($info['path'], 'languages.php') ? 'path ok' : 'path wrong') . "\n";
+
+?>
+--EXPECT--
+0
+0
+no
+null
+0
+0
+0
+yes
+1
+in global
+1
+scope ok
+path ok