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/config/schema/hooks.ts

@@ -37,7 +37,7 @@ export const HookNameSchema = z.enum([
   "delegate-task-retry",
   "prometheus-md-only",
   "sisyphus-junior-notepad",
-  "sisyphus-gpt-hephaestus-reminder",
+  "no-sisyphus-gpt",
   "start-work",
   "atlas",
   "unstable-agent-babysitter",