[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/prompt.zsh

@@ -0,0 +1,3 @@
+# Callers select p10k_config before sourcing this module.
+ZLE_RPROMPT_INDENT=0
+[[ -z "$p10k_config" || ! -f "$p10k_config" ]] || source "$p10k_config"