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_cgo.go

@@ -0,0 +1,74 @@
+//go:build cgo
+
+package main
+
+import (
+	"context"
+
+	"github.com/steveyegge/beads/internal/storage/dolt"
+)
+
+// DoltServerHandle wraps a dolt.Server for CGO builds
+type DoltServerHandle struct {
+	server *dolt.Server
+}
+
+// DoltDefaultSQLPort is the default SQL port for dolt server
+const DoltDefaultSQLPort = dolt.DefaultSQLPort
+
+// DoltDefaultRemotesAPIPort is the default remotesapi port for dolt server
+const DoltDefaultRemotesAPIPort = dolt.DefaultRemotesAPIPort
+
+// StartDoltServer starts a dolt sql-server for federation mode
+func StartDoltServer(ctx context.Context, dataDir, logFile string, sqlPort, remotePort int) (*DoltServerHandle, error) {
+	server := dolt.NewServer(dolt.ServerConfig{
+		DataDir:        dataDir,
+		SQLPort:        sqlPort,
+		RemotesAPIPort: remotePort,
+		Host:           "127.0.0.1",
+		LogFile:        logFile,
+	})
+
+	if err := server.Start(ctx); err != nil {
+		return nil, err
+	}
+
+	return &DoltServerHandle{server: server}, nil
+}
+
+// Stop stops the dolt server
+func (h *DoltServerHandle) Stop() error {
+	if h.server != nil {
+		return h.server.Stop()
+	}
+	return nil
+}
+
+// SQLPort returns the SQL port the server is listening on
+func (h *DoltServerHandle) SQLPort() int {
+	if h.server != nil {
+		return h.server.SQLPort()
+	}
+	return 0
+}
+
+// RemotesAPIPort returns the remotesapi port the server is listening on
+func (h *DoltServerHandle) RemotesAPIPort() int {
+	if h.server != nil {
+		return h.server.RemotesAPIPort()
+	}
+	return 0
+}
+
+// Host returns the host the server is listening on
+func (h *DoltServerHandle) Host() string {
+	if h.server != nil {
+		return h.server.Host()
+	}
+	return ""
+}
+
+// DoltServerAvailable returns true when CGO is available
+func DoltServerAvailable() bool {
+	return true
+}