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-google-calendar/src/google-calendar-source.ts

@@ -1,4 +1,6 @@
-import type { Context, FeedSource } from "@aris/core"
+import type { ActionDefinition, Context, FeedSource } from "@aris/core"
+
+import { UnknownActionError } from "@aris/core"
 
 import type {
 	ApiCalendarEvent,
@@ -63,7 +65,7 @@ const PRIORITY_ALL_DAY = 0.4
  * ```
  */
 export class GoogleCalendarSource implements FeedSource<CalendarFeedItem> {
-	readonly id = "google-calendar"
+	readonly id = "aris.google-calendar"
 
 	private readonly client: GoogleCalendarClient
 	private readonly calendarIds: string[] | undefined
@@ -75,6 +77,14 @@ export class GoogleCalendarSource implements FeedSource<CalendarFeedItem> {
 		this.lookaheadHours = options.lookaheadHours ?? DEFAULT_LOOKAHEAD_HOURS
 	}
 
+	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.fetchAllEvents(context.time)