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/domain/src/sandbox.rs

@@ -145,6 +145,31 @@ pub struct SandboxPlan {
     pub default_posture: Posture,
 }
 
+impl SandboxPlan {
+    /// Builds the ticket-assistant preset: read-only access to the project root,
+    /// denied writes by default, while keeping the assistant run directory
+    /// writable so CLI session metadata/resume files can still function.
+    #[must_use]
+    pub fn project_read_only(project_root: impl Into<String>, run_dir: impl Into<String>) -> Self {
+        let project_root = project_root.into();
+        let run_dir = run_dir.into();
+        let mut allowed = vec![PathGrant {
+            abs_root: project_root,
+            access: PathAccess::RO,
+        }];
+        if !run_dir.trim().is_empty() {
+            allowed.push(PathGrant {
+                abs_root: run_dir,
+                access: PathAccess::RO | PathAccess::RW,
+            });
+        }
+        Self {
+            allowed,
+            default_posture: Posture::Deny,
+        }
+    }
+}
+
 /// Immutable inputs [`compile_sandbox_plan`] interpolates into the absolute roots
 /// it emits. Borrowed: a plan is computed at the launch site, never stored.
 pub struct SandboxContext<'a> {
@@ -465,6 +490,19 @@ mod tests {
         assert!(!g.access.contains(PathAccess::RW));
     }
 
+    #[test]
+    fn project_read_only_preset_allows_project_read_and_run_dir_write_only() {
+        let plan = SandboxPlan::project_read_only(ROOT, "/home/anthony/.ideai/run/assistant");
+        let project = grant(&plan, ROOT).expect("project root granted");
+        assert_eq!(project.access, PathAccess::RO);
+        assert!(!project.access.contains(PathAccess::RW));
+
+        let run = grant(&plan, "/home/anthony/.ideai/run/assistant").expect("run dir granted");
+        assert!(run.access.contains(PathAccess::RO));
+        assert!(run.access.contains(PathAccess::RW));
+        assert_eq!(plan.default_posture, Posture::Deny);
+    }
+
     #[test]
     fn write_and_delete_map_to_rw() {
         let e = eff(