Shorter hook name, disableable via disabled_hooks config, migration added
for backward compatibility. Also forces agent switch to Hephaestus on
Sisyphus + GPT detection. Docs updated with new hook name.
First message variant gate was unconditionally overwriting message.variant
with the fallback chain value (e.g. 'medium' for Hephaestus), ignoring
any variant the user had already selected via OpenCode UI.
Now checks message.variant === undefined before applying the resolved
variant, matching the behavior already used for subsequent messages.
Closes#1861
YeonGyu-Kim