feat(tickets): création d'un assistant IA de ticket (backend + frontend)

Ajoute le chat assistant IA attaché à un ticket (#8).

Backend Rust :
- use cases OpenTicketAssistant/CloseTicketAssistant + tests
- politique d'outils par agent (domain/agent_tool_policy) et policy MCP
- store de contexte assistant + gabarit default_ticket_assistant.md + tests
- events TicketAssistantOpened/Closed
- commandes Tauri open_ticket_chat/close_ticket_chat et câblage state/lib/events

Frontend :
- gateway (ports, adapters ticket + mock, domain)
- hook useTicketAssistant + composant TicketAssistantPanel
- intégration dans TicketDetail

Tests verts : vitest tickets.test.tsx (23), cargo test application::ticket_assistant (1),
infrastructure::assistant_context_store (2).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/infrastructure/tests/assistant_context_store.rs

@@ -0,0 +1,110 @@
+use std::path::PathBuf;
+use std::sync::Arc;
+
+use domain::{
+    AssistantContextProvider, FileSystem, Issue, IssueActor, IssueId, IssueNumber, IssuePriority,
+    IssueStatus, MarkdownDoc, Project, ProjectId, ProjectPath,
+};
+use infrastructure::{FsAssistantContextStore, LocalFileSystem};
+use uuid::Uuid;
+
+struct TempDir(PathBuf);
+
+impl TempDir {
+    fn new() -> Self {
+        let path = std::env::temp_dir().join(format!("idea-assistant-context-{}", Uuid::new_v4()));
+        std::fs::create_dir_all(&path).unwrap();
+        Self(path)
+    }
+
+    fn app_data_dir(&self) -> String {
+        self.0.join("app-data").to_string_lossy().into_owned()
+    }
+
+    fn project_root(&self) -> ProjectPath {
+        ProjectPath::new(self.0.join("project").to_string_lossy().into_owned()).unwrap()
+    }
+}
+
+impl Drop for TempDir {
+    fn drop(&mut self) {
+        let _ = std::fs::remove_dir_all(&self.0);
+    }
+}
+
+fn project(root: ProjectPath) -> Project {
+    Project::new(
+        ProjectId::from_uuid(Uuid::from_u128(1)),
+        "demo",
+        root,
+        domain::remote::RemoteRef::local(),
+        1_000,
+    )
+    .unwrap()
+}
+
+fn issue(number: u64) -> Issue {
+    Issue::new(
+        IssueId::from_uuid(Uuid::from_u128(number as u128)),
+        IssueNumber::new(number).unwrap(),
+        "Clarify onboarding",
+        MarkdownDoc::new("Initial description body"),
+        IssueStatus::Open,
+        IssuePriority::High,
+        MarkdownDoc::new("Carnet notes"),
+        Vec::new(),
+        Vec::new(),
+        IssueActor::User,
+        1_000,
+    )
+    .unwrap()
+}
+
+fn store(app_data_dir: String) -> FsAssistantContextStore {
+    let fs: Arc<dyn FileSystem> = Arc::new(LocalFileSystem::new());
+    FsAssistantContextStore::new(fs, app_data_dir)
+}
+
+#[tokio::test]
+async fn default_context_is_embedded_and_injects_the_ticket() {
+    let tmp = TempDir::new();
+    let project = project(tmp.project_root());
+    let store = store(tmp.app_data_dir());
+
+    let ctx = store
+        .prepare_ticket_assistant_context(&project, &issue(7))
+        .await
+        .unwrap();
+
+    let body = ctx.content.as_str();
+    assert!(body.contains("# Ticket Assistant"));
+    assert!(body.contains("- Ref: #7"));
+    assert!(body.contains("- Title: Clarify onboarding"));
+    assert!(body.contains("Initial description body"));
+    assert!(body.contains("Carnet notes"));
+    assert_eq!(ctx.project_root, project.root.as_str());
+    assert_eq!(ctx.relative_path, "ticket-assistant.md");
+}
+
+#[tokio::test]
+async fn app_data_override_replaces_the_embedded_default() {
+    let tmp = TempDir::new();
+    let app_data_dir = tmp.app_data_dir();
+    let override_path = PathBuf::from(&app_data_dir)
+        .join("assistant")
+        .join("ticket-assistant.md");
+    std::fs::create_dir_all(override_path.parent().unwrap()).unwrap();
+    std::fs::write(&override_path, "# Custom assistant").unwrap();
+
+    let project = project(tmp.project_root());
+    let store = store(app_data_dir);
+    let ctx = store
+        .prepare_ticket_assistant_context(&project, &issue(8))
+        .await
+        .unwrap();
+
+    let body = ctx.content.as_str();
+    assert!(body.starts_with("# Custom assistant"));
+    assert!(!body.contains("You are IdeA's ticket editing assistant"));
+    assert!(body.contains("- Ref: #8"));
+}