fix: gateway install uses --@mosaic:registry flag to preserve npm prefix

Replace temp .npmrc + --userconfig approach with the --@mosaic:registry CLI
flag so the user's real ~/.npmrc (including prefix=/home/user/.npm-global)
is honoured. This eliminates EACCES errors for non-root installs that
relied on a user-level prefix config.

Bump @mosaic/cli and @mosaic/mosaic to 0.0.15.

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

packages/mosaic/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@mosaic/mosaic",
-  "version": "0.0.14",
+  "version": "0.0.15",
   "repository": {
     "type": "git",
     "url": "https://git.mosaicstack.dev/mosaic/mosaic-stack.git",