[ZSH] Extract shared startup modules

Keep the server entrypoint focused on server policy while moving update checks, Antidote loading, interactive defaults, and prompt sourcing into reusable modules.

zsh/lib/antidote.zsh

@@ -0,0 +1,12 @@
+# Callers set plugins_txt and static_file before sourcing this module.
+mkdir -p "${static_file:h}"
+if [[ ! -e ${ZDOTDIR:-$HOME}/.antidote ]]; then
+    git clone https://github.com/mattmc3/antidote.git "${ZDOTDIR:-$HOME}/.antidote"
+fi
+
+zstyle ':completion:*:ssh:*' hosts off
+zstyle ':completion:*:scp:*' hosts off
+zstyle ':completion:*:commands' rehash 1
+
+source "${ZDOTDIR:-$HOME}/.antidote/antidote.zsh"
+antidote load "$plugins_txt" "$static_file"