feat: replace private cas envelope

README.md

@@ -176,8 +176,9 @@ The bootstrap implementation provides:
   `--bearer-secret <secret>`
 - private CAS envelope commands:
   `geth cas add-private <resource> <path>` and
-  `geth cas get-private <resource> <hash> --out <path>`. These use local
-  resource secret epochs and are a prototype envelope, not audited AEAD.
+  `geth cas get-private <resource> <hash> --out <path>`. New writes use an
+  AES-256-GCM envelope bound to the resource and local secret epoch. This does
+  not claim forward secrecy or post-compromise security.
 - local CAS tree objects describe file trees and are stored as CAS blobs
 - local file-root commands: `geth cas root add/list/scan/sync/apply`; root sync
   pulls authorized remote tree metadata and CAS tree bytes into a peer-qualified