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.ts

@@ -1,13 +1,4 @@
-import * as Sentry from '@sentry/nextjs';
-
 export async function register() {
-  if (process.env.NEXT_RUNTIME === 'nodejs') {
-    await import('./sentry.server.config');
-  }
-
-  if (process.env.NEXT_RUNTIME === 'edge') {
-    await import('./sentry.edge.config');
-  }
+  // Instrumentation hook for Next.js
+  // Add any server-side instrumentation here if needed
 }
-
-export const onRequestError = Sentry.captureRequestError;