fix: use gh CLI as git credential helper for private repo auth

Configures gh as git's credential helper at build time so all git
HTTPS operations (clone, push, fetch) authenticate via GH_TOKEN
automatically. Fixes private repo cloning failures on Railway.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: AbirAbbas

Dockerfile

@@ -30,7 +30,11 @@ ENV GIT_AUTHOR_NAME="SWE-AF" \
     GIT_COMMITTER_EMAIL="contact@agentfield.com"
 
 RUN git config --global user.name "SWE-AF" && \
-    git config --global user.email "contact@agentfield.com"
+    git config --global user.email "contact@agentfield.com" && \
+    # Use gh CLI as git credential helper — when GH_TOKEN env var is set at
+    # runtime, all git HTTPS operations (clone, push, fetch) against GitHub
+    # authenticate automatically without URL rewriting.
+    gh auth setup-git
 
 # Install uv for fast package installation
 RUN pip install --no-cache-dir uv