Guard ultrawork variant overrides with SDK metadata

Ultrawork now checks provider SDK metadata before forcing a variant, so unsupported variants are skipped instead of being written into the message state.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin/ultrawork-variant-availability.ts

@@ -0,0 +1,51 @@
+import { normalizeSDKResponse } from "../shared"
+
+type ModelDescriptor = {
+  providerID: string
+  modelID: string
+}
+
+type ProviderListClient = {
+  provider?: {
+    list?: () => Promise<unknown>
+  }
+}
+
+type ProviderModelMetadata = {
+  variants?: Record<string, unknown>
+}
+
+type ProviderListEntry = {
+  id?: string
+  models?: Record<string, ProviderModelMetadata>
+}
+
+type ProviderListData = {
+  all?: ProviderListEntry[]
+}
+
+export async function resolveValidUltraworkVariant(
+  client: unknown,
+  model: ModelDescriptor | undefined,
+  variant: string | undefined,
+): Promise<string | undefined> {
+  if (!model || !variant) {
+    return undefined
+  }
+
+  const providerList = (client as ProviderListClient | null | undefined)?.provider?.list
+  if (typeof providerList !== "function") {
+    return undefined
+  }
+
+  const response = await providerList()
+  const data = normalizeSDKResponse<ProviderListData>(response, {})
+  const providerEntry = data.all?.find((entry) => entry.id === model.providerID)
+  const variants = providerEntry?.models?.[model.modelID]?.variants
+
+  if (!variants) {
+    return undefined
+  }
+
+  return Object.hasOwn(variants, variant) ? variant : undefined
+}