feat(attachments): fondation pipeline durable d'attachments agent/chat + sandbox-safe (#154, QA verte)

Socle #154 : entité attachment, store, contrat DTO et routage session. Modules : domain/chat_attachment, application/chat_attachments, infrastructure/chat_attachments, app-tauri (commands/lib), backend dto.

crates/app-tauri/src/lib.rs

@@ -341,6 +341,7 @@ pub fn run() {
             commands::launch_agent,
             commands::change_agent_profile,
             commands::agent_send,
+            commands::import_chat_attachments,
             commands::cancel_resume,
             commands::set_resume_at,
             commands::interrupt_agent,