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/constants.rs

@@ -0,0 +1,16 @@
+//! Shared constants used across multiple modules.
+
+pub const GITEA_ADMIN_USER: &str = "gitea_admin";
+
+pub const MANAGED_NS: &[&str] = &[
+    "data",
+    "devtools",
+    "ingress",
+    "lasuite",
+    "matrix",
+    "media",
+    "monitoring",
+    "ory",
+    "storage",
+    "vault-secrets-operator",
+];