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/federation_nocgo.go

@@ -0,0 +1,34 @@
+//go:build !cgo
+
+package main
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+)
+
+var federationCmd = &cobra.Command{
+	Use:     "federation",
+	GroupID: "sync",
+	Short:   "Manage peer-to-peer federation (requires CGO)",
+	Long: `Federation commands require CGO and the Dolt storage backend.
+
+This binary was built without CGO support. To use federation features:
+  1. Use pre-built binaries from GitHub releases, or
+  2. Build from source with CGO enabled
+
+Federation enables synchronized issue tracking across multiple Gas Towns,
+each maintaining their own Dolt database while sharing updates via remotes.`,
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Println("Federation requires CGO and Dolt backend.")
+		fmt.Println("")
+		fmt.Println("This binary was built without CGO support. To use federation:")
+		fmt.Println("  1. Download pre-built binaries from GitHub releases")
+		fmt.Println("  2. Or build from source with CGO enabled")
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(federationCmd)
+}