Add bos-update + replicate the dev zsh shell

bos-update: one command that updates both BOS channels — pacman -Syu (snap-pac
snapshotted) and bakery update --all — best-effort so one failing doesn't abort
the other. Baked into the live env and skel.

Shell: match the dev laptop's zsh. Ship Powerlevel10k + zsh-autosuggestions,
zsh-history-substring-search and zsh-syntax-highlighting, sourced from the distro
packages (no oh-my-zsh framework) in the correct order, plus the dev .p10k.zsh.
Powerlevel10k is AUR-only, so it's republished to [breadway] via
packaging/powerlevel10k + a CI workflow (builds libgit2 + gitstatus from source),
same pattern as bibata / zen-browser-bin. skel/.zshrc keeps the BOS QoL aliases
and pywal palette import, with `update` aliased to bos-update.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/profiledef.sh

@@ -23,4 +23,5 @@ file_permissions=(
     ["/usr/local/bin/bos-session"]="0:0:755"
     ["/usr/local/bin/bos-keybinds"]="0:0:755"
     ["/usr/local/bin/bos-welcome"]="0:0:755"
+    ["/usr/local/bin/bos-update"]="0:0:755"
 )