refactor(ai): remove default command and enforce strict window context

- Remove `/ai set default-provider` command and related autocomplete logic
- Remove global `wins_get_ai()` window accessor function
- Enforce strict requirement that `/ai model`, `/ai switch`, `/ai models`, and `/ai clear` commands must be executed from within an active AI chat window
- Update `/ai start` to require `<provider>` argument

BREAKING CHANGE: `/ai model`, `/ai switch`, `/ai models`, and `/ai clear` commands now require an active AI window context. The `/ai set default-provider` command has been removed.

src/ui/window_list.c

@@ -867,24 +867,6 @@ wins_get_prune_wins(void)
     return result;
 }
 
-ProfAiWin*
-wins_get_ai(void)
-{
-    GList* curr = values;
-
-    while (curr) {
-        ProfWin* window = curr->data;
-        if (window->type == WIN_AI) {
-            ProfAiWin* aiwin = (ProfAiWin*)window;
-            assert(aiwin->memcheck == PROFAIWIN_MEMCHECK);
-            return aiwin;
-        }
-        curr = g_list_next(curr);
-    }
-
-    return NULL;
-}
-
 gboolean
 wins_ai_exists(ProfAiWin* const aiwin)
 {

src/ui/window_list.h

@@ -63,7 +63,6 @@ ProfPrivateWin* wins_get_private(const char* const fulljid);
 ProfPluginWin* wins_get_plugin(const char* const tag);
 ProfXMLWin* wins_get_xmlconsole(void);
 ProfVcardWin* wins_get_vcard(void);
-ProfAiWin* wins_get_ai(void);
 gboolean wins_ai_exists(ProfAiWin* const aiwin);
 
 void wins_close_plugin(char* tag);