The dolt storage backend requires CGO due to its gozstd dependency.
This change makes the dolt backend optional using build tags, allowing
`go install` to work on Windows where CGO is disabled by default.
Changes:
- Add BackendFactory registration pattern to factory package
- Create factory_dolt.go with `//go:build cgo` constraint that
registers the dolt backend only when CGO is available
- Update init.go to use factory instead of direct dolt import
- When dolt backend is requested without CGO, provide helpful error
message directing users to pre-built binaries
The sqlite backend (default) works without CGO and covers the majority
of use cases. Users who need dolt can either:
1. Use pre-built binaries from GitHub releases
2. Enable CGO by installing a C compiler
Fixes#1116
