ci: enable turborepo remote cache for all Node.js pipelines

Connect to self-hosted turbo cache at turbo.mosaicstack.dev.
Convert lint/typecheck/test/build steps to use pnpm turbo with
remote cache env vars, removing manual build-shared steps since
turbo handles the dependency graph automatically.

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

turbo.json

@@ -1,5 +1,6 @@
 {
   "$schema": "https://turbo.build/schema.json",
+  "remoteCache": {},
   "tasks": {
     "prisma:generate": {
       "cache": false