fix(gemini): bump native provider aux default to gemini-3.6-flash

The native Gemini provider profile's default_aux_model and the curated
model picker catalog were still pinned to gemini-3.5-flash, a stale
generation now superseded by gemini-3.6-flash (documented GA). Bump
both so the auxiliary-task default and the picker stay in sync with
the current model.

Contract test asserts the durable lockstep invariant only
(default_aux_model is a member of _PROVIDER_MODELS["gemini"]) rather
than pinning either side to a frozen model-name string, so it doesn't
need updating on the next model-generation bump.

hermes_cli/models.py

@@ -290,7 +290,7 @@ _PROVIDER_MODELS: dict[str, list[str]] = {
     "gemini": [
         "gemini-3.1-pro-preview",
         "gemini-3-pro-preview",
-        "gemini-3.5-flash",
+        "gemini-3.6-flash",
         "gemini-3.1-flash-lite-preview",
     ],
     "zai": [