feat(observability): gate Sentry tracing spans behind SENTRY_TRACING

Spans are now created and exported to Sentry only when SENTRY_TRACING is enabled (default off), keeping 24/7 operation within the free-tier span quota. Error monitoring stays always-on. Includes OpenSpec change gate-sentry-spans (proposal, design, specs, tasks).

Co-authored-by: Cursor <cursoragent@cursor.com>

src/instrumentation/sentry.ts

@@ -1,4 +1,5 @@
 import * as Sentry from "@sentry/node";
+import { isTracingEnabled } from "./tracing.js";
 
 const dsn = process.env.SENTRY_DSN?.trim();
 
@@ -8,8 +9,13 @@ export function initSentry(): void {
   Sentry.init({
     dsn,
     environment: process.env.NODE_ENV ?? "development",
-    tracesSampleRate: process.env.NODE_ENV === "production" ? 0.1 : 1,
     integrations: [Sentry.httpIntegration(), Sentry.expressIntegration()],
+    // Tracing/spans are gated by SENTRY_TRACING (default off) to stay within
+    // the Sentry free-tier span quota during 24/7 operation. Error monitoring
+    // is always on.
+    ...(isTracingEnabled()
+      ? { tracesSampleRate: process.env.NODE_ENV === "production" ? 0.1 : 1 }
+      : {}),
   });
 
   process.on("unhandledRejection", (reason) => {