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/calendar-context.ts

@@ -0,0 +1,24 @@
+import type { ContextKey } from "@aris/core"
+
+import { contextKey } from "@aris/core"
+
+import type { CalDavEventData } from "./types.ts"
+
+/**
+ * Calendar context for downstream sources.
+ *
+ * Provides a snapshot of the user's upcoming CalDAV 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: CalDavEventData[]
+	/** Next upcoming event, if any */
+	nextEvent: CalDavEventData | null
+	/** Whether the user has any events today */
+	hasTodayEvents: boolean
+	/** Total number of events today */
+	todayEventCount: number
+}
+
+export const CalDavCalendarKey: ContextKey<CalendarContext> = contextKey("caldavCalendar")