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/shared/migration/hook-names.ts

@@ -5,6 +5,8 @@ export const HOOK_NAME_MAP: Record<string, string | null> = {
   "anthropic-auto-compact": "anthropic-context-window-limit-recovery",
   "sisyphus-orchestrator": "atlas",
 
+  "sisyphus-gpt-hephaestus-reminder": "no-sisyphus-gpt",
+
   // Removed hooks (v3.0.0) - will be filtered out and user warned
   "empty-message-sanitizer": null,
 }