ai-stack: state "skip = stay fully local" on the prompt line itself

The instruction was only in explanatory text a few lines above the actual
prompt (prompt_text "Cloud providers to add []:") — easy to miss once
that's scrolled past, especially since the bracketed default shows empty
but doesn't say what empty means. User feedback: the screen itself should
say it, not just text above it. Now reads "Cloud providers to add (blank =
skip, stay fully local):".

services/ai-stack.sh

@@ -73,7 +73,7 @@ install_ai-stack() {
     echo ""
 
     local CLOUD_CHOICES=""
-    prompt_text "Cloud providers to add []:" "" CLOUD_CHOICES
+    prompt_text "Cloud providers to add (blank = skip, stay fully local):" "" CLOUD_CHOICES
 
     # Parallel arrays: display name, OpenAI-compatible base URL, and entered key
     declare -a CLOUD_NAMES=() CLOUD_URLS=() CLOUD_KEYS=()