Fix sweepStaleSessions() lock_guard type after lock became recursive_mutex

Merging upstream TCPServer.h changed lock from std::mutex to
std::recursive_mutex (the ConsoleServer self-deadlock fix); this call
site still hardcoded std::lock_guard<std::mutex>, which no longer
compiles since lock_guard cannot bind to a different mutex type. Uses
the same class-template-argument-deduction form every other lock_guard
in this file already uses.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

TCPServer.cpp

@@ -108,7 +108,7 @@ namespace core {
       // below would deadlock on this non-recursive mutex.
       std::vector<TCPSession *> stale;
       {
-	 std::lock_guard<std::mutex> guard(lock);
+	 std::lock_guard guard(lock);
 	 for (auto &s : sessions)
 	    if (!s->hasReceivedData && (now - s->acceptedAt) > GRACE_PERIOD)
 	       stale.push_back(s.get());