Add a CLI illustration to the README

keep has no web UI (API + CLI only), so there's no "server screenshot"
in the way wisp/npm-statuspage have one. Instead, a styled terminal
image showing a real round trip: identity registration, a push, a
read-only grant, and that read-only recipient correctly blocked from
pushing. Matches the project family's dark-terminal visual style.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -5,6 +5,11 @@ A self-hosted, end-to-end encrypted secrets store for deploy scripts.
 decrypted locally — the server never sees plaintext values, and never
 sees the key that would let it.
 
+![keep is a CLI, not a web app — no server UI to screenshot. This is a full round trip: register an identity, push a vault, grant a second identity read-only access, and watch it get correctly blocked from writing.](./docs/cli-example.png)
+
+`keep` has no web UI by design — it's API + CLI only, so there's nothing
+server-side to show beyond the terminal above.
+
 ## Why
 
 Across a small family of self-hosted projects, every repo tends to have