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>

instrumentation-client.ts

@@ -1,2 +1,2 @@
-// Sentry client SDK disabled to reduce bundle size (~400KB).
-// To re-enable, restore the @sentry/nextjs import and withSentryConfig in next.config.ts.
+// Client-side instrumentation hook for Next.js
+// Add any client-side instrumentation here if needed