Implement insert and edit sessions (#7)

README.md

@@ -36,6 +36,8 @@ the rollback/commit contract are documented in
 [`docs/recipient-policies.md`](docs/recipient-policies.md).
 Typed list/show/find/decrypted-grep models and secret presentation selection are
 documented in [`docs/read-domains.md`](docs/read-domains.md).
+Insert modes, concurrency-safe edit sessions, and the secure CLI editor-file
+boundary are documented in [`docs/write-domains.md`](docs/write-domains.md).
 
 ## Project layout