feat(core): add FeedSource interface

Unifies DataSource and ContextProvider into a single interface that
forms a dependency graph. Sources declare dependencies on other sources
and can provide context, feed items, or both.

Deprecates DataSource, ContextProvider, and ContextBridge.

Co-authored-by: Ona <no-reply@ona.com>

packages/aris-core/src/index.ts

@@ -5,7 +5,10 @@ export { contextKey, contextValue } from "./context"
 // Feed
 export type { FeedItem } from "./feed"
 
-// Data Source
+// Feed Source
+export type { FeedSource } from "./feed-source"
+
+// Data Source (deprecated - use FeedSource)
 export type { DataSource } from "./data-source"
 
 // Context Provider