fix: set git default branch to master in Dockerfile to suppress hint spam

Tests that create temp git repos produce thousands of lines of
"Using 'master' as the name for the initial branch" hints that
bury actual test failures in agent output.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/Dockerfile

@@ -109,7 +109,8 @@ RUN groupadd -r huskies \
     && chown -R huskies:huskies /usr/local/cargo /usr/local/rustup \
     && chown -R huskies:huskies /app \
     && mkdir -p /workspace/target /app/target \
-    && chown huskies:huskies /workspace/target /app/target
+    && chown huskies:huskies /workspace/target /app/target \
+    && git config --global init.defaultBranch master
 
 # ── Entrypoint ───────────────────────────────────────────────────────
 # Validates required env vars (GIT_USER_NAME, GIT_USER_EMAIL) and