feat(write): event delete + WRITE_CALENDAR foundation (v1.1)

First slice of milestone 2 (write support), per the new plan in
docs/superpowers/plans/2026-06-11-03-write-support.md:

- Delete from the event detail screen with confirmation; recurring events
  choose "only this event" (cancelled exception via CONTENT_EXCEPTION_URI,
  series survives) or "all events in the series" (Events-row delete)
- WRITE_CALENDAR in the manifest; onboarding requests read+write in one
  system dialog but only read gates the app — declining write keeps it
  usable read-only. v1.0 installs get a contextual write request on their
  first delete
- CALENDAR_ACCESS_LEVEL is read into CalendarSource.canModifyContents;
  read-only calendars (WebCal, birthdays, …) show no write actions. The
  no-op placeholder Edit button is removed until edit ships (v1.3)
- Onboarding copy drops the now-false "read-only" claim (DE+EN)
- Tests: repository delete delegation/error propagation, access-level
  mapping; FakeCalendarDataSource grows write ops

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -7,6 +7,24 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Write foundation (milestone 2, slice 1): Calendula can now **delete events**.
+  - Delete action on the event detail screen, with a confirmation dialog;
+    recurring events choose between "Only this event" (a cancelled exception,
+    so the rest of the series survives) and "All events in the series"
+  - `WRITE_CALENDAR` permission: onboarding asks for read+write in one system
+    dialog, but only read access is required — declining write keeps the app
+    fully usable read-only. Existing v1.0 installs are asked for the write
+    upgrade in place, on their first delete
+  - Read-only calendars (WebCal subscriptions, birthday calendars, …) are
+    detected via `CALENDAR_ACCESS_LEVEL` and show no edit/delete actions at all
+
+### Changed
+- Onboarding copy no longer claims "read-only"; it now says your data stays on
+  the device (still no internet permission, still zero telemetry)
+- The placeholder Edit button on the detail screen (a no-op since v0.4) is
+  removed until editing ships in a later slice
+
 ## [1.0.0] — 2026-06-11
 
 First public release. Calendula is a read-only, Material 3 Expressive calendar