secrets: regenerate the module sealed-box cross-check fixture

The manager module's seal now runs over the audited tweetnacl-sealedbox-js (mesh-catalog
anthropic-manager) rather than a hand-transcribed NaCl. Regenerate the fixture's sealed value
from that new seal() over the same node key pair and plaintext, so the fixture is the new
library's output. crypto_box_seal is randomised, so the blob differs; the wire format does
not. TestModuleSealedBoxOpensInGo still opens it under box.OpenAnonymous and recovers the
plaintext, proving TS(tweetnacl-sealedbox-js) to Go interop.

Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF

internal/secrets/testdata/module-sealedbox-fixture.json

@@ -3,5 +3,5 @@
   "managerPublicKey": "rJZ9OSnuCcU5MNi8iV0EK8c5nYN+Cx5A+q+miIIIoUc=",
   "managerPrivateKey": "wGHx9hpbO1pyvLiw8oGwi31LBce3HscDiGhXpNU+wl4=",
   "plaintext": "rt-a-refresh-token-only-the-manager-may-read",
-  "sealed": "yKcpcuBD68n84vFEUufVd1lFCng72BbtyT9/40NCVgjyldBa68pQSpiym0qRVthasb/K21u+HywAgk4saJDAABTpm6E3MeyATSdDoLTz/mNR2p0lcDKE2sSDEI8="
+  "sealed": "/fI4OGzdLLQnhwv1IagCiS8DNhsjiaPdJTaLjZYBayxHa5xLZ5T74+00yxKHToAoMGimQ0pCrz5ykQPp6YPsSFUSO8Oujz48f1tl/xRyRtkbulVBBC6ylS0KAAM="
 }