feat(agents): reintroduce isGpt5_5Model for per-version prompt routing

The GPT_NATIVE_SISYPHUS_RE regex already matches gpt-5.5 (and future
5.6+), which is correct for shared behavior. However, gpt-5.5 now has
its own prompt family separate from gpt-5.4, so we need a narrower
check to route exclusively to the gpt-5-5 variants before falling
through to the regex-matched gpt-5-4 path.

The regex stays as the catch-all for future versions; isGpt5_5Model
is the precise-match guard for the current release.

src/agents/types.ts

@@ -86,6 +86,11 @@ export function isGptNativeSisyphusModel(model: string): boolean {
   return GPT_NATIVE_SISYPHUS_RE.test(modelName);
 }
 
+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 isGpt5_3CodexModel(model: string): boolean {
   const modelName = extractModelName(model).toLowerCase();
   return modelName.includes("gpt-5.3-codex") || modelName.includes("gpt-5-3-codex");