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/ai/ai_client.h

@@ -106,15 +106,6 @@ GList* ai_list_providers(void);
  */
 gchar* ai_providers_find(const char* const search_str, gboolean previous, void* context);
 
-/**
- * Find a model name for autocomplete.
- * @param search_str The search string
- * @param previous Whether to go to previous match
- * @param context ProfAiWin* for the current session
- * @return Model name, or NULL if not found
- */
-gchar* ai_models_find(const char* const search_str, gboolean previous, void* context);
-
 /**
  * Reset the provider autocomplete state.
  * Called from cmd_ac_reset() to clear autocomplete state.