Draft: Deepen schedule preparation session use case

[jjoonleo]

Summary

Refs #535.

This draft PR implements one coherent vertical slice of the shallow use-case consolidation work:

• adds SchedulePreparationSessionUseCase as the workflow boundary for active schedule preparation sessions
• moves early-start state, timed-preparation snapshot save/restore/clear, alarm-prompt validation, start dedupe, and finish cleanup behind that workflow interface
• reduces ScheduleBloc constructor dependencies from the separate session/snapshot/prompt/start/finish wrappers to one workflow dependency
• updates CONTEXT.md with resolved Schedule Preparation Session terminology
• adds plans/issue-535-use-case-classification.md classifying every current domain use case as keep, deepen, consolidate, or delete

Validation

• flutter test test/domain/use-cases/schedule_preparation_session_use_case_test.dart
• flutter test test/presentation/app/bloc/schedule/schedule_bloc_test.dart
• flutter test test/presentation/alarm/screens/preparation_flow_widget_test.dart
• flutter analyze

Remaining Checklist

This is draft because issue #535 is broader than this one family.

• Migrate any non-ScheduleBloc callers off the deleted-category wrappers.
• Delete obsolete shallow wrappers after caller migration is complete.
• Consolidate preparation/template CRUD into one workflow-level use case.
• Consolidate calendar date/week/month loading into one calendar workflow boundary.
• Revisit shallow user stream/load wrappers after auth/account workflows are clarified.

Generated Injectable files were refreshed locally with dart run build_runner build --delete-conflicting-outputs; generated *.config.dart files are ignored by this repo and are not part of the tracked diff.