Clarify remote manifest recovery

internal/artifacts/current_state_commit.go

@@ -147,7 +147,7 @@ func readCurrentStateControlObject(ctx context.Context, store storage.ObjectStor
 func decodeCommittedManifest(ctx context.Context, data []byte) (*manifest.Manifest, error) {
 	m, err := (&manifest.LocalStore{}).LoadReader(ctx, bytes.NewReader(data))
 	if err != nil {
-		return nil, fmt.Errorf("committed session manifest decode failed: %w", err)
+		return nil, fmt.Errorf("committed session manifest decode failed: %w; republish this session to replace unsupported remote state", err)
 	}
 	return m, nil
 }

internal/artifacts/remote_commit_test.go

@@ -92,6 +92,19 @@ func TestLoadCurrentStateUsesMetadataFromOpenedObjectVersion(t *testing.T) {
 	}
 }
 
+func TestDecodeCommittedManifestGuidesRemoteRecovery(t *testing.T) {
+	_, err := decodeCommittedManifest(context.Background(), []byte(`{"session_id":"2026-05-03"}`))
+	if err == nil {
+		t.Fatal("decodeCommittedManifest() error = nil, want unsupported schema failure")
+	}
+	if !strings.Contains(err.Error(), "schema_version is required") || !strings.Contains(err.Error(), "republish this session") {
+		t.Fatalf("decodeCommittedManifest() error = %v, want schema and republish guidance", err)
+	}
+	if strings.Contains(err.Error(), "local state") {
+		t.Fatalf("decodeCommittedManifest() error = %v, must not describe remote state as local", err)
+	}
+}
+
 func TestLoadCurrentStateRejectsPointerCommitIdentityMismatch(t *testing.T) {
 	store := &storage.FakeBackend{}
 	prefix := testCurrentSessionPrefix()