Add global lock to serialize CI runs across branches

Multiple simultaneous test runs compete for PostgreSQL connections,
causing spurious errors. Global flock ensures one branch runs at a time.

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

ci-runner.sh

@@ -31,6 +31,11 @@ if ! flock -n 200; then
   exit 0
 fi
 
+# Serialize all CI runs to avoid DB connection contention
+GLOBAL_LOCK="/tmp/tsharps-ci-global.lock"
+exec 201>"$GLOBAL_LOCK"
+flock 201
+
 echo "=== TSHARPS CI Runner ==="
 echo "Branch: $BRANCH"
 echo "Commit: $COMMIT"