fix(quickjs): use unref() on cleanup interval to prevent hang

Added `unref()` call to QuickJS cleanup interval timer to allow
Node.js process to exit even with active interval.

**Issue**: QuickJS sandbox cleanup interval runs every 15 seconds,
preventing process exit during coverage runs and test completion.

**Solution**: Call `unref()` on setInterval to mark it as not
preventing process exit. Process can now cleanly terminate when
all active work completes.

**Impact**: Coverage runs now complete successfully in ~60s instead
of hanging indefinitely.

Test results with fix:
- All unit tests pass
- Coverage collection completes
- Process exits cleanly after tests

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: danieliser

src/sandbox/quickjs-runtime.ts

@@ -374,6 +374,10 @@ export class QuickJSSandbox {
     this.cleanupInterval = setInterval(() => {
       this.cleanupIdleWorkers();
     }, this.idleTimeout / 2);
+    // Prevent interval from keeping process alive during test cleanup
+    if (this.cleanupInterval.unref) {
+      this.cleanupInterval.unref();
+    }
   }
 
   private cleanupIdleWorkers(): void {