chore: sync stale release-please manifest for loopover-engine 3.15.0

packages/loopover-engine/package.json is 3.15.0 but
.release-please-manifest.json still recorded 3.14.1, so the
release-manifest:sync:check step of validate-code fails on every branch
built off current main. Run the sanctioned npm run release-manifest:sync
to align the manifest, unblocking CI.

Author: RealDiligent

.release-please-manifest.json

@@ -1,6 +1,6 @@
 {
   "packages/loopover-mcp": "3.14.1",
-  "packages/loopover-engine": "3.14.1",
+  "packages/loopover-engine": "3.15.0",
   "packages/loopover-miner": "3.14.1",
   "packages/loopover-ui-kit": "1.2.0"
 }