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/context/context_data.phpt

@@ -0,0 +1,54 @@
+--TEST--
+Context: set/get/has and magic property access for plugin data storage
+--FILE--
+<?php
+require '/var/www/app/context.php';
+
+$templates = new Templates(base: '/tmp/b.php', page: '/tmp/p.php', list: '/tmp/l.php');
+$ctx = new Context(
+    contentDir: '/tmp/content',
+    templates: $templates,
+    requestPath: 'blog/post',
+    hasTrailingSlash: true
+);
+
+// Public readonly properties
+echo $ctx->requestPath . "\n";
+echo $ctx->contentDir . "\n";
+echo ($ctx->hasTrailingSlash ? 'true' : 'false') . "\n";
+
+// set/get/has
+$ctx->set('langPrefix', '/fr');
+echo $ctx->get('langPrefix') . "\n";
+echo ($ctx->has('langPrefix') ? 'yes' : 'no') . "\n";
+echo ($ctx->has('missing') ? 'yes' : 'no') . "\n";
+
+// get with default
+echo $ctx->get('nope', 'fallback') . "\n";
+
+// Magic __get / __set
+$ctx->customKey = 'magic-value';
+echo $ctx->customKey . "\n";
+
+// Magic __get for undefined key returns null
+echo ($ctx->undefined ?? 'null') . "\n";
+
+// set and magic access are the same store
+$ctx->set('shared', 'data');
+echo $ctx->shared . "\n";
+
+$ctx->magic2 = 'x';
+echo $ctx->get('magic2') . "\n";
+?>
+--EXPECT--
+blog/post
+/tmp/content
+true
+/fr
+yes
+no
+fallback
+magic-value
+null
+data
+x