feat: ship streaming disabled by default — opt-in via config

Streaming is now off by default for both CLI and gateway. Users opt in:

CLI (config.yaml):
  display:
    streaming: true

Gateway (config.yaml):
  streaming:
    enabled: true

This lets early adopters test streaming while existing users see zero
change. Once we have enough field validation, we flip the default to
true in a subsequent release.

hermes_cli/config.py

@@ -202,6 +202,7 @@ DEFAULT_CONFIG = {
         "resume_display": "full",
         "bell_on_complete": False,
         "show_reasoning": False,
+        "streaming": False,
         "skin": "default",
     },