feat(gateway): honor MOSAIC_GATEWAY_SKIP_NPM_INSTALL to skip registry reinstall

Lets dev/offline installs keep an already-present global @mosaicstack/gateway
(e.g. a build-from-source `install.sh --dev`) instead of overwriting it with
the registry @latest build during `mosaic wizard` / `mosaic gateway install`.

Refs #675

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RMoEx7hfdFGjUiCHuN1RRi

packages/mosaic/src/stages/gateway-config.ts

@@ -294,7 +294,12 @@ export async function gatewayConfigStage(
   }
 
   // Install the gateway npm package on first install or after failure.
-  if (!opts.skipInstall && !daemonRunning) {
+  // MOSAIC_GATEWAY_SKIP_NPM_INSTALL=1 forces a skip even without opts.skipInstall:
+  // used by dev/offline installs where @mosaicstack/gateway is already present
+  // globally (e.g. a build-from-source `install.sh --dev`) and must not be
+  // overwritten by the registry @latest build.
+  const skipNpmInstall = opts.skipInstall || process.env['MOSAIC_GATEWAY_SKIP_NPM_INSTALL'] === '1';
+  if (!skipNpmInstall && !daemonRunning) {
     installGatewayPackage();
   }