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

internal/storage/factory/factory.go

@@ -9,10 +9,20 @@ import (
 
 	"github.com/steveyegge/beads/internal/configfile"
 	"github.com/steveyegge/beads/internal/storage"
-	"github.com/steveyegge/beads/internal/storage/dolt"
 	"github.com/steveyegge/beads/internal/storage/sqlite"
 )
 
+// BackendFactory is a function that creates a storage backend
+type BackendFactory func(ctx context.Context, path string, opts Options) (storage.Storage, error)
+
+// backendRegistry holds registered backend factories
+var backendRegistry = make(map[string]BackendFactory)
+
+// RegisterBackend registers a storage backend factory
+func RegisterBackend(name string, factory BackendFactory) {
+	backendRegistry[name] = factory
+}
+
 // Options configures how the storage backend is opened
 type Options struct {
 	ReadOnly    bool
@@ -40,9 +50,15 @@ func NewWithOptions(ctx context.Context, backend, path string, opts Options) (st
 			return sqlite.NewWithTimeout(ctx, path, opts.LockTimeout)
 		}
 		return sqlite.New(ctx, path)
-	case configfile.BackendDolt:
-		return dolt.New(ctx, &dolt.Config{Path: path, ReadOnly: opts.ReadOnly})
 	default:
+		// Check if backend is registered (e.g., dolt with CGO)
+		if factory, ok := backendRegistry[backend]; ok {
+			return factory(ctx, path, opts)
+		}
+		// Provide helpful error for dolt on systems without CGO
+		if backend == configfile.BackendDolt {
+			return nil, fmt.Errorf("dolt backend requires CGO (not available on this build); use sqlite backend or install from pre-built binaries")
+		}
 		return nil, fmt.Errorf("unknown storage backend: %s (supported: sqlite, dolt)", backend)
 	}
 }

internal/storage/factory/factory_dolt.go

@@ -0,0 +1,17 @@
+//go:build cgo
+
+package factory
+
+import (
+	"context"
+
+	"github.com/steveyegge/beads/internal/configfile"
+	"github.com/steveyegge/beads/internal/storage"
+	"github.com/steveyegge/beads/internal/storage/dolt"
+)
+
+func init() {
+	RegisterBackend(configfile.BackendDolt, func(ctx context.Context, path string, opts Options) (storage.Storage, error) {
+		return dolt.New(ctx, &dolt.Config{Path: path, ReadOnly: opts.ReadOnly})
+	})
+}