feat(agents): add gpt-5.5 native sisyphus support

Route GPT-5.5 through the existing GPT-5.4-native Sisyphus and Hephaestus prompt family while keeping one shared model-family helper.

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

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

src/agents/types.ts

@@ -84,6 +84,15 @@ export function isGpt5_4Model(model: string): boolean {
   return modelName.includes("gpt-5.4") || modelName.includes("gpt-5-4");
 }
 
+export function isGpt5_5Model(model: string): boolean {
+  const modelName = extractModelName(model).toLowerCase();
+  return modelName.includes("gpt-5.5") || modelName.includes("gpt-5-5");
+}
+
+export function isGptNativeSisyphusModel(model: string): boolean {
+  return isGpt5_4Model(model) || isGpt5_5Model(model);
+}
+
 export function isGpt5_3CodexModel(model: string): boolean {
   const modelName = extractModelName(model).toLowerCase();
   return modelName.includes("gpt-5.3-codex") || modelName.includes("gpt-5-3-codex");