fix: enable building on Windows without CGO (#1117)

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

cmd/bd/init.go

@@ -16,7 +16,7 @@ import (
 	"github.com/steveyegge/beads/internal/configfile"
 	"github.com/steveyegge/beads/internal/git"
 	"github.com/steveyegge/beads/internal/storage"
-	"github.com/steveyegge/beads/internal/storage/dolt"
+	"github.com/steveyegge/beads/internal/storage/factory"
 	"github.com/steveyegge/beads/internal/storage/sqlite"
 	"github.com/steveyegge/beads/internal/syncbranch"
 	"github.com/steveyegge/beads/internal/types"
@@ -299,7 +299,7 @@ With --stealth: configures per-repository git settings for invisible beads usage
 		if backend == configfile.BackendDolt {
 			// Dolt uses a directory, not a file
 			storagePath = filepath.Join(beadsDir, "dolt")
-			store, err = dolt.New(ctx, &dolt.Config{Path: storagePath})
+			store, err = factory.New(ctx, backend, storagePath)
 		} else {
 			storagePath = initDBPath
 			store, err = sqlite.New(ctx, storagePath)