Add a script for re-recording every demo a page embeds

Changing demo/settings.tape, or anything about how lazygit looks, dates
every recording at once, and re-recording them one at a time means
running the recorder fifteen times and pasting fifteen new URLs.
Attachment URLs say nothing about where they came from, so there is also
nothing to tell you which demo a video in the README is of.

Name the demo in a comment above each video. GitHub drops the comment
when it renders the page, so it costs the reader nothing, and it gives
us a way back from a page to the demo that produced it. Then walk those
comments, re-record each demo and rewrite the URL below it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/rerecord_demos.sh

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+demo/rerecord_demos.sh "$@"