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/hooks/hooks_add_apply.phpt

@@ -0,0 +1,38 @@
+--TEST--
+Hooks: apply passes value through, add registers filters that chain in order
+--FILE--
+<?php
+require '/var/www/app/hooks.php';
+
+// No filters registered — passthrough
+echo Hooks::apply(Hook::PROCESS_CONTENT, 'hello') . "\n";
+echo Hooks::apply(Hook::TEMPLATE_VARS, 'foo') . "\n";
+
+// Register a filter
+Hooks::add(Hook::PROCESS_CONTENT, fn($v) => strtoupper($v));
+echo Hooks::apply(Hook::PROCESS_CONTENT, 'hello') . "\n";
+
+// Filters chain: second receives output of first
+Hooks::add(Hook::PROCESS_CONTENT, fn($v) => $v . '!');
+echo Hooks::apply(Hook::PROCESS_CONTENT, 'hello') . "\n";
+
+// Other hooks are unaffected
+echo Hooks::apply(Hook::TEMPLATE_VARS, 'foo') . "\n";
+
+// Extra args are passed through to each filter
+Hooks::add(Hook::TEMPLATE_VARS, fn($v, $extra) => "$v:$extra");
+echo Hooks::apply(Hook::TEMPLATE_VARS, 'x', 'ctx') . "\n";
+
+// Non-string values pass through too
+$arr = ['a' => 1];
+echo Hooks::apply(Hook::CONTEXT_READY, $arr) === $arr ? 'array passthrough' : 'fail';
+echo "\n";
+?>
+--EXPECT--
+hello
+foo
+HELLO
+HELLO!
+foo
+x:ctx
+array passthrough