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.

assets/oh-my-opencode.schema.json

@@ -91,7 +91,7 @@
           "delegate-task-retry",
           "prometheus-md-only",
           "sisyphus-junior-notepad",
-          "sisyphus-gpt-hephaestus-reminder",
+          "no-sisyphus-gpt",
           "start-work",
           "atlas",
           "unstable-agent-babysitter",