fix(hooks): allow feature branches in contributor workflow (#850)

The pre-push hook now detects when an `upstream` remote is configured
and allows feature branches for the fork contribution workflow.

Previously, the hook blocked all non-main branches, which prevented
pushing PR branches to forks. Now the blocking logic checks for an
upstream remote - if present, it skips the block and allows the push.

The check wraps the blocking logic (rather than early-out) so that
any future additions to the hook will still apply to contributor
workflows.

Fixes #848

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

.githooks/pre-push

@@ -15,17 +15,22 @@ while read local_ref local_sha remote_ref remote_sha; do
       # Allowed branches
       ;;
     *)
-      echo "ERROR: Invalid branch for Gas Town agents."
-      echo ""
-      echo "Blocked push to: $branch"
-      echo ""
-      echo "Allowed branches:"
-      echo "  main        - Crew workers push here directly"
-      echo "  polecat/*   - Polecat working branches"
-      echo "  beads-sync  - Beads synchronization"
-      echo ""
-      echo "Do NOT create PRs. Push to main or let Refinery merge polecat work."
-      exit 1
+      # Allow feature branches when contributing to upstream (fork workflow).
+      # If an 'upstream' remote exists, this is a contribution setup where
+      # feature branches are needed for PRs. See: #848
+      if ! git remote get-url upstream &>/dev/null; then
+        echo "ERROR: Invalid branch for Gas Town agents."
+        echo ""
+        echo "Blocked push to: $branch"
+        echo ""
+        echo "Allowed branches:"
+        echo "  main        - Crew workers push here directly"
+        echo "  polecat/*   - Polecat working branches"
+        echo "  beads-sync  - Beads synchronization"
+        echo ""
+        echo "Do NOT create PRs. Push to main or let Refinery merge polecat work."
+        exit 1
+      fi
       ;;
   esac
 done