docs: drop shell-quotes from REGISTRY_USER in .env.example

Gitea secrets and the .env loader both treat the value literally; the
single quotes were misleading and risked being copied verbatim into the
stored secret.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env.example

@@ -6,7 +6,7 @@ CONTAINER_ENGINE=docker
 
 # ── Docker Registry ──
 REGISTRY=kcr.kollect.biz
-REGISTRY_USER='robot$kollect-tools+ci-builder'
+REGISTRY_USER=robot$kollect-tools+ci-builder
 REGISTRY_PASS=harbor-robot-password
 IMAGE_TAG=latest