fix: omit Authorization header in listTags when no access token

Sending an empty Bearer token made GitHub reject matching-refs on public
repos with 401. Drop the header entirely when no token is set so
unauthenticated requests succeed, and default $accessToken to '' so
empty() is safe on an uninitialized adapter.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: loks0n

src/VCS/Adapter/Git/GitHub.php

@@ -32,7 +32,7 @@ class GitHub extends Git
 
     protected string $endpoint = 'https://api.github.com';
 
-    protected string $accessToken;
+    protected string $accessToken = '';
 
     protected string $jwtToken;
 
@@ -809,7 +809,8 @@ public function listBranches(string $owner, string $repositoryName, int $perPage
     public function listTags(string $owner, string $repositoryName, string $search = ''): array
     {
         $url = "/repos/$owner/$repositoryName/git/matching-refs/tags/";
-        $response = $this->call(self::METHOD_GET, $url, ['Authorization' => "Bearer $this->accessToken"]);
+        $headers = empty($this->accessToken) ? [] : ['Authorization' => "Bearer $this->accessToken"];
+        $response = $this->call(self::METHOD_GET, $url, $headers);
 
         $statusCode = $response['headers']['status-code'] ?? 0;
         $responseBody = $response['body'] ?? [];