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/doctor/federation.go

@@ -1,3 +1,5 @@
+//go:build cgo
+
 package doctor
 
 import (

cmd/bd/doctor/federation_nocgo.go

@@ -0,0 +1,53 @@
+//go:build !cgo
+
+package doctor
+
+// CheckFederationRemotesAPI returns N/A when CGO is not available.
+func CheckFederationRemotesAPI(path string) DoctorCheck {
+	return DoctorCheck{
+		Name:     "Federation remotesapi",
+		Status:   StatusOK,
+		Message:  "N/A (requires CGO)",
+		Category: CategoryFederation,
+	}
+}
+
+// CheckFederationPeerConnectivity returns N/A when CGO is not available.
+func CheckFederationPeerConnectivity(path string) DoctorCheck {
+	return DoctorCheck{
+		Name:     "Peer Connectivity",
+		Status:   StatusOK,
+		Message:  "N/A (requires CGO)",
+		Category: CategoryFederation,
+	}
+}
+
+// CheckFederationSyncStaleness returns N/A when CGO is not available.
+func CheckFederationSyncStaleness(path string) DoctorCheck {
+	return DoctorCheck{
+		Name:     "Sync Staleness",
+		Status:   StatusOK,
+		Message:  "N/A (requires CGO)",
+		Category: CategoryFederation,
+	}
+}
+
+// CheckFederationConflicts returns N/A when CGO is not available.
+func CheckFederationConflicts(path string) DoctorCheck {
+	return DoctorCheck{
+		Name:     "Federation Conflicts",
+		Status:   StatusOK,
+		Message:  "N/A (requires CGO)",
+		Category: CategoryFederation,
+	}
+}
+
+// CheckDoltServerModeMismatch returns N/A when CGO is not available.
+func CheckDoltServerModeMismatch(path string) DoctorCheck {
+	return DoctorCheck{
+		Name:     "Dolt Mode",
+		Status:   StatusOK,
+		Message:  "N/A (requires CGO)",
+		Category: CategoryFederation,
+	}
+}