secretbox: add AES-256-GCM sealing of small secrets

Moved out of Hammerhead's internal package so Sandbar can use it too.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -1,6 +1,6 @@
 # Reef
 
-A shared library for Hammerhead, Blacktip, and Bamboo
+A shared library for Hammerhead, Blacktip, Bamboo and Sandbar
 
 Shared types and building blocks for the sharks stack: [Hammerhead](https://git.veillax.com/Veillax/hammerhead) (control plane), [Blacktip](https://git.veillax.com/Veillax/blacktip) (edge) and [Bamboo](https://git.veillax.com/Veillax/bamboo) (site agent).
 
@@ -15,6 +15,7 @@ Reef holds the code all three programs agree on, so each can be built and releas
 | `netguard` | Refuses internal addresses for anything that connects out on a user's behalf (webhooks, mail delivery), and dials the address it checked |
 | `docker` | Lists running containers through the Docker Engine API, with no SDK |
 | `dotenv` | Loads a `.env` file into the environment without overriding real variables |
+| `secretbox` | AES-256-GCM sealing of small secrets (`enc1:` values) before they are written to a database |
 
 ## Using it