Extracts the five domain types out of src/core/model.go into a new
src/domain package (job.go, record.go, config.go). The unexported
nextDue field is promoted to NextDue so it is accessible from core.
All references across src/core, src/gui, and cmd/gosentry are updated
to use domain.TypeName. src/core/model.go is reduced to a bare package
declaration. Windows and Linux cross-compilation both pass; all tests
remain green.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
mixeme
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