feat(ai): add model caching, settings, and commands

- Introduce model caching with persistence to preferences
- Add provider default model and custom settings management
- Implement `/ai switch`, `/ai models`, and improve `/ai start`
- Add model name autocomplete for chat commands
- Update command definitions and help text
- Add unit tests for new functionality

src/ai/ai_client.h

@@ -17,12 +17,15 @@ typedef struct ai_message_t
  */
 typedef struct ai_provider_t
 {
-    gchar* name;       /* Provider name (e.g., "openai", "perplexity") */
-    gchar* api_url;    /* API endpoint URL */
-    gchar* org_id;     /* Optional organization ID */
-    gchar* project_id; /* Optional project ID (for some providers) */
-    GList* models;     /* List of available models (gchar*) */
-    gint32 ref_count;  /* Reference count (atomic) */
+    gchar* name;           /* Provider name (e.g., "openai", "perplexity") */
+    gchar* api_url;        /* API endpoint URL */
+    gchar* org_id;         /* Optional organization ID */
+    gchar* project_id;     /* Optional project ID (for some providers) */
+    gchar* default_model;  /* Default model for this provider */
+    GHashTable* settings;  /* Extensible per-provider settings (e.g., tools=enabled, search=disabled) */
+    GList* models;         /* Cached models (gchar*) */
+    gboolean models_fresh; /* Whether models cache is current */
+    gint32 ref_count;      /* Reference count (atomic) */
 } AIProvider;
 
 /**
@@ -97,6 +100,15 @@ 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);
+
 /**
  * Get the API key for a provider.
  * @param provider_name The provider name
@@ -111,6 +123,51 @@ gchar* ai_get_provider_key(const gchar* provider_name);
  */
 void ai_set_provider_key(const gchar* provider_name, const gchar* api_key);
 
+/**
+ * Set the default model for a provider.
+ * @param provider_name The provider name
+ * @param model The default model name
+ */
+void ai_set_provider_default_model(const gchar* provider_name, const gchar* model);
+
+/**
+ * Get the default model for a provider.
+ * @param provider_name The provider name
+ * @return The default model name (caller must not free), or NULL if not set
+ */
+const gchar* ai_get_provider_default_model(const gchar* provider_name);
+
+/**
+ * Set a custom setting for a provider.
+ * @param provider_name The provider name
+ * @param setting The setting key (e.g., "tools", "search")
+ * @param value The setting value
+ */
+void ai_set_provider_setting(const gchar* provider_name, const gchar* setting, const gchar* value);
+
+/**
+ * Get a custom setting for a provider.
+ * @param provider_name The provider name
+ * @param setting The setting key
+ * @return The setting value (caller must free), or NULL if not set
+ */
+gchar* ai_get_provider_setting(const gchar* provider_name, const gchar* setting);
+
+/**
+ * Fetch available models from a provider's API.
+ * @param provider_name The provider name
+ * @param user_data User data (ProfAiWin* for UI display)
+ * @return TRUE if the request was successfully queued, FALSE otherwise
+ */
+gboolean ai_fetch_models(const gchar* provider_name, gpointer user_data);
+
+/**
+ * Check if models cache is fresh for a provider.
+ * @param provider_name The provider name
+ * @return TRUE if models are fresh, FALSE otherwise
+ */
+gboolean ai_models_are_fresh(const gchar* provider_name);
+
 /* ========================================================================
  * Session Management
  * ======================================================================== */