chore: remove Sentry integration

Remove @sentry/nextjs and all related files since it was never actively used.
- Delete sentry.server.config.ts, sentry.edge.config.ts
- Delete sentry-example-page and sentry-example-api routes
- Clean up instrumentation.ts, global-error.tsx, middleware.ts
- Remove Sentry env vars from env.example and docs
- Update CLAUDE.md, copilot-instructions.md, PRODUCTION_READINESS.md

Middleware bundle reduced from 86KB to 34.8KB (-51KB).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

package.json

@@ -55,7 +55,6 @@
     "@next/bundle-analyzer": "^15.1.7",
     "@prisma/client": "^5.22.0",
     "@react-three/fiber": "^9.5.0",
-    "@sentry/nextjs": "^10.36.0",
     "@shadergradient/react": "^2.4.20",
     "@swc/helpers": "^0.5.19",
     "@tiptap/extension-color": "^3.15.3",