[hmirin]

Summary

Add medium and large macOS agenda widgets matching Until's menubar timeline.

• The medium widget highlights the active or imminent next event and shows up to three following events, adapting to available height.
• The large widget shows today's agenda including dimmed completed events. Row spacing adapts to the number of events; crowded days retain the highlighted event in the visible range.
• The footer shows the undisplayed event count, or “終わり” / “End” when every event is visible.
• Both sizes follow the menubar's overlapping-event and imminent-event preferences, hide the current-time line while the highlighted event is active, and open Until when clicked.

The widget reads a filtered agenda snapshot from the app's shared container without receiving OAuth credentials. Packaging embeds and signs the extension with the app. This PR also enables pull-request CI and documents PR-based development.

Validation

• Nine targeted agenda tests passed locally, including full-day window selection.
• SwiftLint and signed app packaging passed.
• Native SwiftUI renders inspected for large full-day, overflow, completed-day, and medium layouts; Japanese footer labels render correctly.
• Local development build 1.1.2 (24) installed with matching signing identity and configuration; app launch and widget extension registration verified.
• Desktop refresh and click behavior on this build have not been visually verified.
• CI passed for the preceding commit; the latest push triggers a new run.