Introduce ExportKit automation architecture

[CodyBontecou]

Summary

• Introduces shared ExportKit and ExportAutomationKit architecture for export path planning, format rendering, destination writing, preview building, plugins, scheduling, and pending export automation.
• Adapts Health.md export flows and previews to the new reusable export abstractions across iOS, iPadOS, and macOS.
• Expands unit coverage for ExportKit primitives, Health export plugins/previews, scheduling date math, pending exports, APNs preflight, and macOS export execution.
• Adds adoption/architecture docs plus the scheduled APNs worker contract.

Testing

• git diff --check origin/main...HEAD

Note

• This branch includes the existing local commit 071a4997 Restore pricing analytics worker instrumentation that was already ahead of origin/main before branching.