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/domain/tests/entities.rs

@@ -4,7 +4,8 @@
 mod helpers;
 
 use domain::{
-    Agent, AgentManifest, AgentOrigin, AgentProfile, AgentTemplate, ContextInjection, DomainError,
+    Agent, AgentManifest, AgentOrigin, AgentProfile, AgentTemplate, ChatAttachment,
+    ChatAttachmentError, ChatAttachmentId, ChatAttachmentSourceKind, ContextInjection, DomainError,
     ManifestEntry, MarkdownDoc, ProfileId, Project, ProjectPath, PtySize, RemoteRef,
     SessionStrategy, Skill, SkillId, SkillRef, SkillScope, SshAuth, TemplateId, TemplateVersion,
 };
@@ -19,6 +20,23 @@ fn template_id() -> TemplateId {
     TemplateId::from_uuid(Uuid::from_u128(7))
 }
 
+fn chat_attachment() -> ChatAttachment {
+    ChatAttachment {
+        id: ChatAttachmentId::new("attach-1").unwrap(),
+        project_id: domain::ProjectId::from_uuid(Uuid::from_u128(1)),
+        agent_id: domain::AgentId::from_uuid(Uuid::from_u128(2)),
+        session_id: domain::SessionId::from_uuid(Uuid::from_u128(3)),
+        filename: "note.txt".to_owned(),
+        mime: "text/plain".to_owned(),
+        size_bytes: 12,
+        source_kind: ChatAttachmentSourceKind::LocalFile,
+        storage_path: "agent-chat/session/attach-1-note.txt".to_owned(),
+        readable_path: "/project/.ideai/attachments/agent-chat/session/attach-1-note.txt"
+            .to_owned(),
+        created_at: 123,
+    }
+}
+
 // ---------------------------------------------------------------------------
 // ProjectPath
 // ---------------------------------------------------------------------------
@@ -52,6 +70,26 @@ fn project_path_rejects_empty() {
     assert!(matches!(err, DomainError::EmptyField { .. }));
 }
 
+// ---------------------------------------------------------------------------
+// Chat attachments
+// ---------------------------------------------------------------------------
+
+#[test]
+fn chat_attachment_metadata_accepts_agent_chat_storage_path() {
+    assert!(chat_attachment().validate().is_ok());
+}
+
+#[test]
+fn chat_attachment_metadata_rejects_storage_path_outside_agent_chat_namespace() {
+    let mut attachment = chat_attachment();
+    attachment.storage_path = "tickets/1/file.txt".to_owned();
+
+    assert!(matches!(
+        attachment.validate(),
+        Err(ChatAttachmentError::InvalidStoragePath(_))
+    ));
+}
+
 // ---------------------------------------------------------------------------
 // Project (also exercises the Clock/IdGenerator port fakes for determinism)
 // ---------------------------------------------------------------------------