server: do not advertise a connect code without the tunnel

--connect-code on its own had no effect other than being served in
/props, so the Web UI showed a remote access code that no tunnel was
serving. Clear it when --connect is off, and say so once at startup.

The teardown comment claimed that terminate() closes the child stdout.
It does not: the log thread leaves fgets() only once every writer of
that pipe is gone, so a descendant of llama-connect would block the
join forever. Describe the real contract instead.

Also restore the --reasoning-preserve default in the generated docs.

tools/server/server.cpp

@@ -184,6 +184,10 @@ int llama_server(common_params & params, int argc, char ** argv) {
             return 1;
         }
         params.server_connect_code = server_connect::resolve_code(params.server_connect_code);
+    } else if (!params.server_connect_code.empty()) {
+        // the code is only meaningful while the tunnel runs, and /props keys off it being set
+        SRV_WRN("%s", "--connect-code is ignored without --connect\n");
+        params.server_connect_code.clear();
     }
 
     // note: this is guaranteed to out-live ctx_http and tools