refactor(workspace): enforce layered crate boundaries

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

@@ -0,0 +1,7 @@
+use crate::{ProviderCapability, ProviderDescriptor, ProviderHealth};
+
+pub trait ProviderEligibilityPolicy: Send + Sync {
+    fn is_eligible(&self, provider: &ProviderDescriptor, capability: ProviderCapability) -> bool {
+        provider.health != ProviderHealth::Unavailable && provider.supports(capability)
+    }
+}