feat: add actions to FeedSource interface

Add listActions() and executeAction() to FeedSource for write
operations back to external services. Actions use arktype schemas
for input validation via StandardSchemaV1.

- ActionDefinition type with optional input schema
- FeedEngine routes actions with existence and ID validation
- Source IDs use reverse-domain format (aris.location, aris.tfl)
- LocationSource: update-location action with schema validation
- TflSource: set-lines-of-interest action with lineId validation
- No-op implementations for sources without actions

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

packages/aris-core/src/index.ts

@@ -2,6 +2,10 @@
 export type { Context, ContextKey } from "./context"
 export { contextKey, contextValue } from "./context"
 
+// Actions
+export type { ActionDefinition } from "./action"
+export { UnknownActionError } from "./action"
+
 // Feed
 export type { FeedItem } from "./feed"