refactor(workspace): enforce layered crate boundaries

crates/aether-provider/core/src/capability.rs

@@ -0,0 +1,11 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum ProviderCapability {
+    Chat,
+    Responses,
+    Embeddings,
+    Images,
+    Video,
+    Files,
+}