refactor: rename service references arcadia-app → arcadia-core

The Phoenix auth/identity/tenancy backend repo is being renamed
arcadia-app → arcadia-core (its primary OTP app is already arcadia_core).
Updates prose, doc paths, and git.sky-ai.com repo URLs. Deliberately
leaves the Rust crate arcadia-app-client and host arcadia-app.internal
(handled separately), and the kept namespace (issuer/release "arcadia").

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/routes/sso.tsx

@@ -580,7 +580,7 @@ function IdpEditorDialog({
               id="idp-callback"
               value={callbackUrl}
               onChange={(e) => setCallbackUrl(e.target.value)}
-              placeholder="https://your-arcadia-app/api/v1/auth/saml/callback"
+              placeholder="https://your-arcadia-core/api/v1/auth/saml/callback"
               className="font-mono text-xs"
               data-action="idp-form-callback"
             />