feat(poc): add terrors PoC crate scaffold and error types

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/crates/arbiter-server/tests/user_agent.rs

@@ -2,5 +2,7 @@ mod common;
 
 #[path = "user_agent/auth.rs"]
 mod auth;
+#[path = "user_agent/sdk_client.rs"]
+mod sdk_client;
 #[path = "user_agent/unseal.rs"]
 mod unseal;