hookify docs: warn that Stop fires every turn — SessionEnd is the once-per-session hook

One-line docs correction for hookify's hook-authoring guidance: wherever the `Stop` hook is mentioned as a candidate for end-of-session logic, state explicitly:

> **`Stop` fires at the end of EVERY assistant response, not once per session.** For once-per-session behavior (a summary, a receipt, cleanup), use `SessionEnd`.

Observed failure: a receipt-printing hook wired to `Stop` with the intent "print once when the session wraps up" — it printed on every assistant turn (runaway physical receipt printer, in this case). Same intent, wrong fire-condition. It's a naming/mental-model trap rather than a code bug, so the warning belongs at the point where `Stop` is first introduced.

The issue does not name a file; first locate hookify's hook-authoring guidance and the first introduction of the Stop hook. Add the warning that Stop fires after every assistant response and that SessionEnd is for once-per-session behavior, then verify the guidance clearly distinguishes those fire conditions.

Written by the indexing model from the issue text.