fix(build): add CGO build constraints for Dolt-dependent files

The dolthub/gozstd dependency requires CGO. Several files were importing
the dolt package without build constraints, causing CI failures when
building with CGO_ENABLED=0 for Linux, FreeBSD, and Android.

Changes:
- Add //go:build cgo to federation.go and doctor/federation.go
- Create dolt_server_cgo.go/nocgo.go to abstract dolt.Server usage
- Create federation_nocgo.go with stub command explaining CGO requirement
- Create doctor/federation_nocgo.go with stub health checks
- Update daemon.go to use the dolt server abstraction

Federation and Dolt-specific features are unavailable in non-CGO builds.
Users are directed to pre-built binaries from GitHub releases.

Fixes v0.49.0 CI failure.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/dolt_server_nocgo.go

@@ -0,0 +1,50 @@
+//go:build !cgo
+
+package main
+
+import (
+	"context"
+	"errors"
+)
+
+// DoltServerHandle is a stub for non-CGO builds
+type DoltServerHandle struct{}
+
+// DoltDefaultSQLPort is the default SQL port for dolt server
+const DoltDefaultSQLPort = 3306
+
+// DoltDefaultRemotesAPIPort is the default remotesapi port for dolt server
+const DoltDefaultRemotesAPIPort = 50051
+
+// ErrDoltRequiresCGO is returned when dolt features are requested without CGO
+var ErrDoltRequiresCGO = errors.New("dolt backend requires CGO; use pre-built binaries from GitHub releases or enable CGO")
+
+// StartDoltServer returns an error in non-CGO builds
+func StartDoltServer(ctx context.Context, dataDir, logFile string, sqlPort, remotePort int) (*DoltServerHandle, error) {
+	return nil, ErrDoltRequiresCGO
+}
+
+// Stop is a no-op stub
+func (h *DoltServerHandle) Stop() error {
+	return nil
+}
+
+// SQLPort returns 0 in non-CGO builds
+func (h *DoltServerHandle) SQLPort() int {
+	return 0
+}
+
+// RemotesAPIPort returns 0 in non-CGO builds
+func (h *DoltServerHandle) RemotesAPIPort() int {
+	return 0
+}
+
+// Host returns empty string in non-CGO builds
+func (h *DoltServerHandle) Host() string {
+	return ""
+}
+
+// DoltServerAvailable returns false when CGO is not available
+func DoltServerAvailable() bool {
+	return false
+}