feat(direnv): warn user if direnv not found

Instead of silently not installing the shell hooks, issue a warning (then keep doing nothing).

plugins/direnv/direnv.plugin.zsh

@@ -1,5 +1,8 @@
-# Don't continue if direnv is not found
+# If direnv is not found, don't continue and print a warning
-command -v direnv &>/dev/null || return
+if ! command -v direnv &>/dev/null; then
+  echo "Warning: direnv not found. Please install direnv and ensure it's in your PATH before using this plugin."
+  return
+fi
 
 _direnv_hook() {
   trap -- '' SIGINT;