refactor: rename @crema/arcadia-client → @crema/arcadia-core-client

Disambiguates the Phoenix/auth client lib from lib-arcadia-agents-client.
Dir lib-arcadia-client → lib-arcadia-core-client; alias updated in
tsconfig paths, vite config, app.css @source, imports, CI and docs.

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

src/login-form.tsx

@@ -7,7 +7,7 @@ import {
   useArcadiaClient,
   type ArcadiaAuthTokens,
   type ArcadiaUser,
-} from "@crema/arcadia-client";
+} from "@crema/arcadia-core-client";
 
 export interface LoginResult {
   tokens: ArcadiaAuthTokens;