refactor(config): make EditorState/ConfirmDeleteState generic over the artifact type

Review feedback: string widening dropped the compile-time guard at the
write/delete boundary right where the Codex panel (the next task) starts
writing into this state. Threading <TType extends string = CcArtifactType>
through EditorState/EditorModal and ConfirmDeleteState/ConfirmDeleteModal
keeps CcConfig.tsx's write/delete calls checked against CcArtifactType by
default while still letting a Codex-typed caller instantiate its own union
later, removing the two `as CcArtifactType` casts.

client/src/pages/CcConfig.tsx

@@ -426,7 +426,7 @@ export function CcConfig() {
 
   const handleSave = useCallback(
     async (args: {
-      type: string;
+      type: CcArtifactType;
       targetScope: "user" | "project" | "auto-memory";
       name: string | undefined;
       content: string;
@@ -434,7 +434,7 @@ export function CcConfig() {
     }) => {
       const result: CcMutationResult = await api.ccConfig.write({
         scope: args.targetScope,
-        type: args.type as CcArtifactType,
+        type: args.type,
         name: args.name,
         content: args.content,
         project: args.project,
@@ -456,7 +456,7 @@ export function CcConfig() {
     try {
       const result = await api.ccConfig.delete({
         scope: confirmDelete.scope,
-        type: confirmDelete.type as CcArtifactType,
+        type: confirmDelete.type,
         name: confirmDelete.name,
         project: confirmDelete.project,
       });