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/package.json

@@ -0,0 +1,15 @@
+{
+	"name": "@aris/source-apple-calendar",
+	"version": "0.0.0",
+	"type": "module",
+	"main": "src/index.ts",
+	"types": "src/index.ts",
+	"scripts": {
+		"test": "bun test ."
+	},
+	"dependencies": {
+		"@aris/core": "workspace:*",
+		"ical.js": "^2.1.0",
+		"tsdav": "^2.1.7"
+	}
+}