refactor(hooks): rename sisyphus-gpt-hephaestus-reminder to no-sisyphus-gpt

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.

src/plugin/chat-message.ts

@@ -81,7 +81,7 @@ export function createChatMessageHandler(args: {
     await hooks.keywordDetector?.["chat.message"]?.(input, output)
     await hooks.claudeCodeHooks?.["chat.message"]?.(input, output)
     await hooks.autoSlashCommand?.["chat.message"]?.(input, output)
-    await hooks.sisyphusGptHephaestusReminder?.["chat.message"]?.(input)
+    await hooks.noSisyphusGpt?.["chat.message"]?.(input, output)
     if (hooks.startWork && isStartWorkHookOutput(output)) {
       await hooks.startWork["chat.message"]?.(input, output)
     }