update skill step 11 aborts on a valid snapshot under set -e

[realmarcin]

From the Codex review of #216.

Where: .claude/skills/update-xmech-page/SKILL.md step 11.

When every entry is allowed, grep -v selects no lines and exits 1. The extra=$(…) assignment takes that status, so a shell running with set -e (bash) or ERR_EXIT (zsh) exits before du and rm, and the snapshot stays.

Fix: build the listing so that "nothing extra" exits 0, for example with a case loop instead of grep -v.