Move store.go, paths.go, and store_test.go from src/core into the new
src/storage package. Update src/scheduler and src/gui to import storage
instead of core for Store/Paths/OpenStore. Empty the moved files in core
to preserve the package declaration for the remaining core symbols.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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