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-source-apple-calendar/src/calendar-source.ts

@@ -1,4 +1,5 @@
-import type { Context, FeedSource } from "@aris/core"
+import type { ActionDefinition, Context, FeedSource } from "@aris/core"
+import { UnknownActionError } from "@aris/core"
 
 import { DAVClient } from "tsdav"
 
@@ -37,7 +38,7 @@ const DEFAULT_LOOK_AHEAD_DAYS = 0
  * ```
  */
 export class CalendarSource implements FeedSource<CalendarFeedItem> {
-	readonly id = "apple-calendar"
+	readonly id = "aris.apple-calendar"
 
 	private readonly credentialProvider: CalendarCredentialProvider
 	private readonly userId: string
@@ -58,6 +59,14 @@ export class CalendarSource implements FeedSource<CalendarFeedItem> {
 		this.injectedClient = options?.davClient ?? null
 	}
 
+	async listActions(): Promise<Record<string, ActionDefinition>> {
+		return {}
+	}
+
+	async executeAction(actionId: string): Promise<void> {
+		throw new UnknownActionError(actionId)
+	}
+
 	async fetchContext(context: Context): Promise<Partial<Context> | null> {
 		const events = await this.fetchEvents(context)
 		if (events.length === 0) {