feat(chat): #155 paste image depuis presse-papier dans le composer custom (QA verte)

- frontend: onPaste sur CustomAgentChatView, détection MIME image, chip/preview d'attachment, envoi via le contrat #154 (adapters/agent, ports)

- backend: import d'image par bytes dans le store attachments (commands, chat_attachments app+infra, dto, ports) + tests

crates/domain/src/ports.rs

@@ -1110,6 +1110,17 @@ pub trait ChatAttachmentStore: Send + Sync {
         import: ChatAttachmentImport,
     ) -> Result<ChatAttachment, ChatAttachmentStoreError>;
 
+    /// Writes in-memory bytes into IdeA-managed durable attachment storage.
+    async fn import_from_bytes(
+        &self,
+        root: &ProjectPath,
+        project_id: ProjectId,
+        agent_id: AgentId,
+        session_id: SessionId,
+        bytes: &[u8],
+        import: ChatAttachmentImport,
+    ) -> Result<ChatAttachment, ChatAttachmentStoreError>;
+
     /// Lists attachments imported for a structured/chat session.
     async fn list_for_session(
         &self,