Implement embedded OpenPGP compatibility (#4)

README.md

@@ -28,6 +28,9 @@ The shared TOML schema, path rules, editor precedence, and HTTPS remote format
 are documented in [`docs/configuration.md`](docs/configuration.md).
 The capability-scoped password-store layout and atomic mutation guarantees are
 documented in [`docs/repository-core.md`](docs/repository-core.md).
+The embedded OpenPGP backend, exported-key model, secret-provider boundary, and
+GnuPG compatibility evidence are documented in
+[`docs/cryptography.md`](docs/cryptography.md).
 
 ## Project layout