stop the run at the end of the rewritten sections

A WIP boundary after section 2 so the finished part can be run start to finish
on its own, without the untouched sections below acting on the machine. It moves
down as each section is worked through and goes away when the walk ends.

Also ignores .setup-progress, which the script writes beside itself and is
per-machine. The exit message names it, because with it in place a second run
skips section 2 and the rewritten part cannot be re-felt from scratch.

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

.gitignore

@@ -51,3 +51,6 @@ src/apps/officer-web/index.gen.html
 
 # Sidecar assets published at install time — copies of files that live in each sidecar's own tree.
 public/plugins/
+
+# Written by machine-setup.sh to record completed steps; per-machine, never shared.
+.setup-progress