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/run.php

@@ -0,0 +1,112 @@
+<?php
+
+// phpt test runner — execute via run.sh, runs inside the container
+
+const GREEN  = "\033[32m";
+const RED    = "\033[31m";
+const YELLOW = "\033[33m";
+const RESET  = "\033[0m";
+const BOLD   = "\033[1m";
+
+$filter = $argv[1] ?? null;
+
+$iterator = new RecursiveIteratorIterator(
+    new RecursiveDirectoryIterator(__DIR__, FilesystemIterator::SKIP_DOTS)
+);
+$files = [];
+foreach ($iterator as $file) {
+    if ($file->getExtension() === 'phpt') {
+        $files[] = $file->getPathname();
+    }
+}
+sort($files);
+
+$pass = 0;
+$fail = 0;
+$skip = 0;
+
+foreach ($files as $file) {
+    $rel  = ltrim(substr($file, strlen(__DIR__), -5), '/');
+
+    if ($filter !== null && !str_contains($rel, $filter)) {
+        continue;
+    }
+
+    $sections = parse_phpt(file_get_contents($file));
+    $title    = trim($sections['TEST'] ?? $rel);
+
+    if (!isset($sections['FILE']) || !isset($sections['EXPECT'])) {
+        echo RED . 'INVALID' . RESET . " $name (missing --FILE-- or --EXPECT--)\n";
+        $fail++;
+        continue;
+    }
+
+    if (isset($sections['SKIPIF'])) {
+        $result = trim(run_php($sections['SKIPIF']));
+        if (str_starts_with($result, 'skip')) {
+            $reason = ltrim(substr($result, 4));
+            echo YELLOW . 'SKIP' . RESET . " $title" . ($reason ? " — $reason" : '') . "\n";
+            $skip++;
+            continue;
+        }
+    }
+
+    $actual   = trim(run_php($sections['FILE']));
+    $expected = trim($sections['EXPECT']);
+
+    if ($actual === $expected) {
+        echo GREEN . 'PASS' . RESET . " $title\n";
+        $pass++;
+    } else {
+        echo RED . 'FAIL' . RESET . " $title\n";
+        show_diff($expected, $actual);
+        $fail++;
+    }
+}
+
+$total = $pass + $fail + $skip;
+echo "\n" . BOLD . "Results: $pass/$total passed" . ($skip ? ", $skip skipped" : '') . RESET . "\n";
+exit($fail > 0 ? 1 : 0);
+
+// ---- helpers ----
+
+function parse_phpt(string $content): array
+{
+    $sections = [];
+    $current  = null;
+    foreach (explode("\n", $content) as $line) {
+        if (preg_match('/^--([A-Z]+)--$/', rtrim($line), $m)) {
+            $current            = $m[1];
+            $sections[$current] = '';
+        } elseif ($current !== null) {
+            $sections[$current] .= $line . "\n";
+        }
+    }
+    return $sections;
+}
+
+function run_php(string $code): string
+{
+    $tmp = tempnam('/tmp', 'phpt_');
+    file_put_contents($tmp, $code);
+    $output = shell_exec("php $tmp 2>&1");
+    unlink($tmp);
+    return $output ?? '';
+}
+
+function show_diff(string $expected, string $actual): void
+{
+    $exp = explode("\n", $expected);
+    $act = explode("\n", $actual);
+    $max = max(count($exp), count($act));
+    for ($i = 0; $i < $max; $i++) {
+        $e = $exp[$i] ?? null;
+        $a = $act[$i] ?? null;
+        if ($e === $a) {
+            echo "    $e\n";
+        } else {
+            if ($e !== null) echo RED   . "  - $e" . RESET . "\n";
+            if ($a !== null) echo GREEN . "  + $a" . RESET . "\n";
+        }
+    }
+}