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/config/preferences.h

@@ -380,4 +380,9 @@ gboolean prefs_ai_remove_models(const char* const provider);
 GList* prefs_ai_get_models(const char* const provider);
 void prefs_free_ai_models(GList* models);
 
+/* AI default model per provider */
+gboolean prefs_ai_set_default_model(const char* const provider, const char* const model);
+gboolean prefs_ai_remove_default_model(const char* const provider);
+char* prefs_ai_get_default_model(const char* const provider);
+
 #endif