feat(ai): persist custom providers to config and move defaults to preferences

Fix /ai set provider <name> <url> not persisting across restarts.

- Add prefs_ai_set_provider(), prefs_ai_remove_provider(),
  prefs_ai_get_provider_url(), prefs_ai_list_providers(),
  prefs_free_ai_providers(), and prefs_ai_get_default_provider_url()
  to preferences.h/c for provider persistence.

- Move default provider URL definitions (openai, perplexity) from
  ai_client.c to preferences.c, following the established pattern
  where defaults are returned as fallbacks when no config value exists.

- Modify ai_add_provider() to persist new/updated providers to config.

- Modify ai_remove_provider() to remove provider from config.

- Add internal _ai_add_provider_nopersist() helper for adding default
  providers and loading from config without double-persisting.

- Update ai_client_init() to load custom providers from config on
  startup, allowing users to override defaults or add new ones.

Users now have full control: default providers remain as fallbacks,
user-added providers persist to config, and users can override or
remove defaults freely.

src/config/preferences.c

@@ -1863,6 +1863,126 @@ prefs_free_ai_models(GList* models)
     }
 }
 
+/* ========================================================================
+ * AI Provider Management
+ * ======================================================================== */
+
+gboolean
+prefs_ai_set_provider(const char* const provider, const char* const url)
+{
+    if (!provider || !url)
+        return FALSE;
+
+    if (!prefs)
+        return FALSE;
+
+    /* Store URL with "_url" suffix to avoid collision with API key storage */
+    g_key_file_set_string(prefs, PREF_GROUP_AI, g_strconcat(provider, "_url", NULL), url);
+    _save_prefs();
+    return TRUE;
+}
+
+gboolean
+prefs_ai_remove_provider(const char* const provider)
+{
+    if (!provider)
+        return FALSE;
+
+    if (!prefs)
+        return FALSE;
+
+    auto_gchar gchar* url_key = g_strconcat(provider, "_url", NULL);
+    if (!g_key_file_has_key(prefs, PREF_GROUP_AI, url_key, NULL)) {
+        return FALSE;
+    }
+
+    g_key_file_remove_key(prefs, PREF_GROUP_AI, url_key, NULL);
+    _save_prefs();
+    return TRUE;
+}
+
+char*
+prefs_ai_get_provider_url(const char* const provider)
+{
+    if (!provider || !prefs)
+        return NULL;
+
+    auto_gchar gchar* key = g_strconcat(provider, "_url", NULL);
+    if (!g_key_file_has_key(prefs, PREF_GROUP_AI, key, NULL)) {
+        return NULL;
+    }
+
+    return g_key_file_get_string(prefs, PREF_GROUP_AI, key, NULL);
+}
+
+GList*
+prefs_ai_list_providers(void)
+{
+    if (!prefs || !g_key_file_has_group(prefs, PREF_GROUP_AI)) {
+        return NULL;
+    }
+
+    GList* result = NULL;
+    gsize len = 0;
+    auto_gcharv gchar** keys = g_key_file_get_keys(prefs, PREF_GROUP_AI, &len, NULL);
+
+    for (gsize i = 0; i < len; i++) {
+        char* key = keys[i];
+        /* Match keys ending with "_url" that are not "_models" suffix */
+        if (g_str_has_suffix(key, "_url") && !g_str_has_suffix(key, "_models_url")) {
+            /* Extract provider name by removing "_url" suffix */
+            gsize key_len = strlen(key);
+            if (key_len > 4) {
+                gchar* provider_name = g_strndup(key, key_len - 4);
+                result = g_list_append(result, provider_name);
+            }
+        }
+    }
+
+    return result;
+}
+
+void
+prefs_free_ai_providers(GList* providers)
+{
+    if (providers) {
+        g_list_free_full(providers, g_free);
+    }
+}
+
+GList*
+prefs_ai_get_providers(void)
+{
+    /* If user has configured any providers, return those */
+    GList* configured = prefs_ai_list_providers();
+    if (configured && g_list_length(configured) > 0) {
+        /* Build result list with name, url pairs */
+        GList* result = NULL;
+        GList* curr = configured;
+        while (curr) {
+            gchar* name = (gchar*)curr->data;
+            gchar* url = prefs_ai_get_provider_url(name);
+            if (url && strlen(url) > 0) {
+                result = g_list_append(result, g_strdup(name));
+                result = g_list_append(result, g_strdup(url));
+            }
+            g_free(url);
+            curr = g_list_next(curr);
+        }
+        prefs_free_ai_providers(configured);
+        return result;
+    }
+    prefs_free_ai_providers(configured);
+
+    /* Nothing configured — return built-in defaults */
+    GList* result = NULL;
+    result = g_list_append(result, g_strdup("openai"));
+    result = g_list_append(result, g_strdup("https://api.openai.com/"));
+    result = g_list_append(result, g_strdup("perplexity"));
+    result = g_list_append(result, g_strdup("https://api.perplexity.ai/"));
+    return result;
+}
+
 // get the preference group for a specific preference
 // for example the PREF_BEEP setting ("beep" in .profrc, see _get_key) belongs
 // to the [ui] section.

src/config/preferences.h

@@ -366,6 +366,14 @@ char* prefs_ai_get_token(const char* const provider);
 GList* prefs_ai_list_tokens(void);
 void prefs_free_ai_tokens(GList* tokens);
 
+/* AI provider management */
+gboolean prefs_ai_set_provider(const char* const provider, const char* const url);
+gboolean prefs_ai_remove_provider(const char* const provider);
+char* prefs_ai_get_provider_url(const char* const provider);
+GList* prefs_ai_list_providers(void);
+void prefs_free_ai_providers(GList* providers);
+GList* prefs_ai_get_providers(void);
+
 /* AI model cache management */
 gboolean prefs_ai_set_models(const char* const provider, const gchar* const* models, gint count);
 gboolean prefs_ai_remove_models(const char* const provider);