zsh plugin: switch to loading ~/.orbstack/shell/init.zsh

Completions are now pre-generated and added to `fpath` in `~/.orbstack/shell/init.zsh`.

Signed-off-by: Danny Lin <danny@kdrag0n.dev>

orb.plugin.zsh

@@ -1,6 +1,2 @@
-# make sure you execute this *after* asdf or other version managers are loaded
-if (( $+commands[orbctl] )); then
-  eval "$(orbctl completion zsh)"
-  compdef _orb orbctl
-  compdef _orb orb
-fi
+# OrbStack: command-line tools and integration
+source ~/.orbstack/shell/init.zsh 2>/dev/null || :