feat: add generic CalDAV calendar data source (#42)

* feat: add generic CalDAV calendar data source

Add @aris/source-caldav package that fetches calendar events from any
CalDAV server via tsdav + ical.js.

- Supports Basic auth and OAuth via explicit authMethod discriminant
- serverUrl provided at construction time, not hardcoded
- Optional timeZone for correct local day boundaries
- Credentials cleared from memory after client login
- Failed calendar fetches logged, not silently dropped
- Login promise cached with retry on failure

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

* fix: deduplicate concurrent fetchEvents calls

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

* fix: timezone-aware signals, low-priority cancelled events

- computeSignals uses startOfDay(timeZone) for 'later today' boundary
- Cancelled events get urgency 0.1, excluded from context inProgress/nextEvent

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

---------

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

packages/aris-source-caldav/src/index.ts

@@ -0,0 +1,15 @@
+export { CalDavCalendarKey, type CalendarContext } from "./calendar-context.ts"
+export { CalDavSource, type CalDavSourceOptions } from "./caldav-source.ts"
+export { parseICalEvents } from "./ical-parser.ts"
+export {
+	AttendeeRole,
+	AttendeeStatus,
+	CalDavEventStatus,
+	type CalDavAlarm,
+	type CalDavAttendee,
+	type CalDavDAVCalendar,
+	type CalDavDAVClient,
+	type CalDavDAVObject,
+	type CalDavEventData,
+	type CalDavFeedItem,
+} from "./types.ts"