fix: first instance always binds port 16620, fallback to auto-port

Ensures Claude Code MCP registration (fixed at 16620) always works.
Second+ instances get OS-assigned ports, discoverable via list_instances().

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

Source/Plugin/McpServer.cpp

@@ -69,7 +69,12 @@ void McpServer::start()
     if (running.load()) return;
     serverThread = std::thread([this]() {
         running.store(true);
-        port = server->bind_to_any_port("127.0.0.1");
+        // Try port 16620 first (Claude Code MCP registration). If taken by
+        // another instance, fall back to OS-assigned port.
+        if (server->bind_to_port("127.0.0.1", 16620))
+            port = 16620;
+        else
+            port = server->bind_to_any_port("127.0.0.1");
         server->listen_after_bind();
         running.store(false);
     });