Maintenance – Merge pr #59 && add integration test

[derpixler]

Adds support for the Zammad Tag API administration scope: list, create, rename and delete tags globally via tag_list endpoints.

Changes

• src/Resource/Tag.php — Added all, create, update, delete URL definitions for admin scope (cherry-picked from Add support for Tag: Administration scope #59 by @christopheg)
• README.md — Updated Tag feature row in the capability table and added admin scope documentation link
• test/ZammadAPIClient/Resource/TagTest.php — Added integration tests for all four admin operations
• examples/tag_admin.php — New example showing tag lifecycle: list → create → rename → delete

How to test
vendor/bin/phpunit --filter "TagTest::testAdmin"

Thanks to @christopheg for the original implementation in #59.