Phase 1: register iwd Agent for passphrase prompts

Export net.connman.iwd.Agent at /net/eiwd/agent and register it via
AgentManager. RequestPassphrase replies are deferred so the UI can
prompt asynchronously; the manager exposes
iwd_manager_set_passphrase_handler for the UI layer to plug in.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/iwd/iwd_manager.h

@@ -3,6 +3,7 @@
 
 #include <Eldbus.h>
 #include <Eina.h>
+#include "iwd_agent.h"
 
 typedef enum {
    IWD_STATE_OFF,
@@ -35,4 +36,10 @@ void iwd_manager_listener_del (Iwd_Manager *m, Iwd_Manager_Cb cb, void *data);
 /* Internal: invoked by sub-objects when their state changes. */
 void iwd_manager_notify (Iwd_Manager *m);
 
+/* The UI installs its passphrase prompt here. The handler must
+ * eventually call iwd_agent_reply()/iwd_agent_cancel() with the request. */
+void iwd_manager_set_passphrase_handler(Iwd_Manager *m,
+                                        Iwd_Agent_Passphrase_Cb cb,
+                                        void *data);
+
 #endif