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/fixtures/minimal-event.ics

@@ -0,0 +1,10 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Test//Test//EN
+BEGIN:VEVENT
+UID:minimal-001@test
+DTSTART:20260115T180000Z
+DTEND:20260115T190000Z
+SUMMARY:Quick Chat
+END:VEVENT
+END:VCALENDAR