feat: add Apple Calendar source package

Add @aris/source-apple-calendar for fetching iCloud
calendar events via CalDAV using tsdav and ical.js.

- CalendarSource implements FeedSource with fetchItems
  and fetchContext for downstream context
- CalendarCredentialProvider interface for token injection
- CalendarDAVClient interface for testability
- iCal parser extracts full event data including
  attendees, alarms, organizer, and recurrence
- Priority based on event proximity to current time

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

packages/aris-source-apple-calendar/src/calendar-context.ts

@@ -0,0 +1,24 @@
+import type { ContextKey } from "@aris/core"
+
+import { contextKey } from "@aris/core"
+
+import type { CalendarEventData } from "./types.ts"
+
+/**
+ * Calendar context for downstream sources.
+ *
+ * Provides a snapshot of the user's upcoming events so other sources
+ * can adapt (e.g. a commute source checking if there's a meeting soon).
+ */
+export interface CalendarContext {
+	/** Events happening right now */
+	inProgress: CalendarEventData[]
+	/** Next upcoming event, if any */
+	nextEvent: CalendarEventData | null
+	/** Whether the user has any events today */
+	hasTodayEvents: boolean
+	/** Total number of events today */
+	todayEventCount: number
+}
+
+export const CalendarKey: ContextKey<CalendarContext> = contextKey("calendar")