feat(federation): add SQL user authentication for peer sync

Merge SQL user authentication with Emma federation sync implementation:

- Add federation_peers table for encrypted credential storage
- Add credentials.go with AES-256-GCM encryption, SHA-256 key derivation
- Extend FederatedStorage interface with credential methods
- Add --user, --password, --sovereignty flags to bd federation add-peer
- Integrate credentials into PushTo/PullFrom/Fetch via withPeerCredentials
- DOLT_REMOTE_USER/PASSWORD env vars protected by mutex for concurrency

Credentials automatically used when syncing with peers that have stored auth.

Continues: bd-wkumz.10, Closes: bd-4p67y

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

internal/storage/dolt/credentials_test.go

@@ -0,0 +1,13 @@
+package dolt
+
+import (
+	"testing"
+
+	"github.com/steveyegge/beads/internal/storage"
+)
+
+func TestDoltStoreImplementsCredentialMethods(t *testing.T) {
+	// Compile-time check that DoltStore implements FederatedStorage with credential methods
+	var _ storage.FederatedStorage = (*DoltStore)(nil)
+	t.Log("DoltStore implements FederatedStorage interface with credential methods")
+}