ref(ai): remove org_id from provider configuration

The org_id field was removed from AIProvider as it was no longer
populated through the command surface. The /ai set org command was
previously removed, leaving org_id as dead code.

This change removes org_id from:
- AIProvider struct definition
- ai_provider_new() and ai_provider_unref()
- ai_add_provider() API signature
- _build_curl_headers() (OpenAI-Organization header)
- cmd_ai_set() and cmd_ai_providers()
- All unit tests

src/ai/ai_client.h

@@ -19,7 +19,6 @@ 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) */
     gchar* default_model;  /* Default model for this provider */
     GHashTable* settings;  /* Extensible per-provider settings (e.g., tools=enabled, search=disabled) */
@@ -66,10 +65,9 @@ AIProvider* ai_get_provider(const gchar* name);
  * Add or update a provider configuration.
  * @param name The provider name
  * @param api_url The API endpoint URL
- * @param org_id Optional organization ID (can be NULL)
  * @return New AIProvider* (caller must unref when done)
  */
-AIProvider* ai_add_provider(const gchar* name, const gchar* api_url, const gchar* org_id);
+AIProvider* ai_add_provider(const gchar* name, const gchar* api_url);
 
 /**
  * Remove a provider by name.