refactor: SDK core modules — error, config, output, constants

Move foundational modules into sunbeam-sdk. All crate-internal references
remain unchanged since these are sibling modules within the SDK crate.

sunbeam-sdk/src/lib.rs

@@ -0,0 +1,19 @@
+#[macro_use]
+pub mod error;
+
+pub mod auth;
+pub mod checks;
+pub mod cluster;
+pub mod config;
+pub mod constants;
+pub mod gitea;
+pub mod images;
+pub mod kube;
+pub mod manifests;
+pub mod openbao;
+pub mod output;
+pub mod pm;
+pub mod secrets;
+pub mod services;
+pub mod update;
+pub mod users;