fix(ai): persistence and command-output fixes

- Persist per-provider default model in [ai] prefs (set/get/remove
  helpers, load on init, clear with provider removal). Previously
  /ai set default-model only updated runtime state.

- Stop re-seeding openai/perplexity into [ai] every time the configured
  provider list is empty. The defaults are now seeded once, guarded by
  a "defaults_initialized" sentinel, so removing every provider does
  not silently restore them.

- /ai providers (no arg) now lists the configured providers with key
  status, instead of the hardcoded openai/perplexity blurb.

- /ai models <provider> (fresh fetch) prints the model names it cached,
  not just a count. Previously the user had to follow up with --cached
  to actually see them.

- /ai remove provider now also clears the persisted default model,
  token, and cached model list, leaving no orphan keys in [ai].

- Drop dead ai_models_find declaration from ai_client.h (no
  implementation in this tree).

src/command/cmd_funcs.c

@@ -11174,30 +11174,24 @@ cmd_ai_clear(ProfWin* window, const char* const command, gchar** args)
 gboolean
 cmd_ai_providers(ProfWin* window, const char* const command, gchar** args)
 {
-    if (args[1] == NULL || g_strcmp0(args[1], "list") != 0) {
-        // List all available providers
-        cons_show("Available AI providers:");
-        cons_show("");
-        cons_show("  openai      - OpenAI API (https://api.openai.com)");
-        cons_show("  perplexity  - Perplexity API (https://api.perplexity.ai)");
-        cons_show("");
-        cons_show("Add custom providers with: /ai set provider <name> <url>");
-        return TRUE;
-    }
-
-    // List configured providers with key status
     cons_show("Configured providers:");
     cons_show("");
 
     GList* providers = ai_list_providers();
+    if (!providers) {
+        cons_show("  (none configured)");
+        cons_show("");
+        cons_show("Add a provider with: /ai set provider <name> <url>");
+        return TRUE;
+    }
+
     for (GList* curr = providers; curr; curr = g_list_next(curr)) {
         AIProvider* provider = curr->data;
-        gchar* key = ai_get_provider_key(provider->name);
+        auto_gchar gchar* key = ai_get_provider_key(provider->name);
         cons_show("  %s", provider->name);
         cons_show("    URL: %s", provider->api_url);
         cons_show("    Key: %s", key ? "configured" : "NOT configured");
         cons_show("");
-        g_free(key);
     }
     g_list_free(providers);