fix(hooks): apply Sisyphus GPT-5.5 variant

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

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

src/hooks/no-sisyphus-gpt/hook.ts

@@ -5,7 +5,7 @@ import {
   resolveRegisteredAgentName,
   updateSessionAgent,
 } from "../../features/claude-code-session-state"
-import { log } from "../../shared"
+import { AGENT_MODEL_REQUIREMENTS, log } from "../../shared"
 import { getAgentConfigKey } from "../../shared/agent-display-names"
 
 const TOAST_TITLE = "NEVER Use Sisyphus with GPT"
@@ -30,6 +30,18 @@ function showToast(ctx: PluginInput, sessionID: string): void {
   })
 }
 
+function getNativeSisyphusGptVariant(model: { providerID: string; modelID: string }): string | undefined {
+  const chain = AGENT_MODEL_REQUIREMENTS["sisyphus"]?.fallbackChain ?? []
+  const exactMatch = chain.find((entry) =>
+    entry.providers.includes(model.providerID) && entry.model === model.modelID
+  )
+  if (exactMatch?.variant !== undefined) {
+    return exactMatch.variant
+  }
+
+  return chain.find((entry) => entry.model === model.modelID)?.variant
+}
+
 export function createNoSisyphusGptHook(ctx: PluginInput) {
   return {
     "chat.message": async (input: {
@@ -43,6 +55,20 @@ export function createNoSisyphusGptHook(ctx: PluginInput) {
       const agentKey = getAgentConfigKey(rawAgent)
       const modelID = input.model?.modelID
 
+      if (
+        agentKey === "sisyphus"
+        && input.model
+        && modelID
+        && isGptNativeSisyphusModel(modelID)
+        && output?.message
+        && output.message.variant === undefined
+      ) {
+        const variant = getNativeSisyphusGptVariant(input.model)
+        if (variant !== undefined) {
+          output.message.variant = variant
+        }
+      }
+
       if (agentKey === "sisyphus" && modelID && isGptModel(modelID) && !isGptNativeSisyphusModel(modelID)) {
         showToast(ctx, input.sessionID)
         input.agent = resolveRegisteredAgentName("hephaestus") ?? "hephaestus"