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>

app/global-error.tsx

@@ -1,6 +1,5 @@
 "use client";
 
-import * as Sentry from "@sentry/nextjs";
 import { useEffect } from "react";
 
 export default function GlobalError({
@@ -11,15 +10,9 @@ export default function GlobalError({
   reset: () => void;
 }) {
   useEffect(() => {
-    // Capture exception in Sentry
-    Sentry.captureException(error);
-    
-    // Log error details to console
-    console.error("Global Error:", error);
-    console.error("Error Name:", error.name);
-    console.error("Error Message:", error.message);
-    console.error("Error Stack:", error.stack);
-    console.error("Error Digest:", error.digest);
+    if (process.env.NODE_ENV === "development") {
+      console.error("Global Error:", error);
+    }
   }, [error]);
 
   return (