refactor: one logging helper LIT\MCP\Log (M6)

Log::failure(context, detail) keeps the '[wp-lit-mcp] <context> failed: <detail>'
format of the nine former error_log() sites; Log::line() covers the change log line.
One phpcs ignore instead of eleven.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/Unit/LogTest.php

@@ -0,0 +1,53 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit;
+
+use LIT\MCP\Log;
+use RuntimeException;
+
+final class LogTest extends TestCase
+{
+    /** @return string What the callback wrote to the error log. */
+    private static function captured(callable $write): string
+    {
+        // error_log() is a PHP internal and cannot be stubbed; redirect it to a temp file instead.
+        $log = (string) tempnam(sys_get_temp_dir(), 'lit-mcp-log');
+        $previous = ini_set('error_log', $log);
+
+        try {
+            $write();
+        } finally {
+            ini_set('error_log', (string) $previous);
+        }
+
+        $content = (string) file_get_contents($log);
+        unlink($log);
+
+        return $content;
+    }
+
+    public function testFailureKeepsThePrefixedFormat(): void
+    {
+        $content = self::captured(static fn () => Log::failure('media usage query', 'Table gone'));
+
+        self::assertStringContainsString('[wp-lit-mcp] media usage query failed: Table gone', $content);
+    }
+
+    public function testFailureLogsAThrowableWithItsTrace(): void
+    {
+        $e = new RuntimeException('kaputt');
+        $content = self::captured(static fn () => Log::failure('lit/x', $e));
+
+        self::assertStringContainsString('[wp-lit-mcp] lit/x failed: RuntimeException: kaputt', $content);
+        self::assertStringContainsString('Stack trace', $content);
+    }
+
+    public function testLineOnlyAddsThePrefix(): void
+    {
+        $content = self::captured(static fn () => Log::line('change log insert failed for a b:1'));
+
+        self::assertStringContainsString('[wp-lit-mcp] change log insert failed for a b:1', $content);
+    }
+}