feat(sprints): modèle de sprints — domaine, use-cases, persistance et surfaces (backend)

Ticket #10 — introduction du modèle de sprints côté backend.

- Domaine : nouvel agrégat Sprint (sprint.rs), IDs, événements et invariants ;
  rattachement des issues à un sprint (issue.rs) et ports associés.
- Application : use-cases sprints (application/src/sprints) + erreurs dédiées.
- Infrastructure : store de sprints (infrastructure/src/sprints.rs), adaptation
  du store d'issues et exposition MCP via orchestrator/mcp/tickets.rs.
- app-tauri : commandes, state et events pour piloter les sprints depuis l'UI.

Tests domaine/application/infra/app-tauri verts (sprint_usecases, sprint_store,
issue_store, mcp_server).

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

crates/domain/src/lib.rs

@@ -58,6 +58,7 @@ pub mod remote;
 pub mod sandbox;
 pub mod session_limit;
 pub mod skill;
+pub mod sprint;
 pub mod template;
 pub mod terminal;
 
@@ -71,7 +72,7 @@ pub use error::DomainError;
 
 pub use ids::{
     AgentId, IssueId, LayoutId, NodeId, ProfileId, ProjectId, ScheduleId, SessionId, SkillId,
-    TabId, TaskId, TemplateId, WindowId,
+    SprintId, TabId, TaskId, TemplateId, WindowId,
 };
 
 pub use project::{Project, ProjectPath};
@@ -116,6 +117,11 @@ pub use issue::{
     IssueVersion,
 };
 
+pub use sprint::{
+    validate_sprint_ordering, Sprint, SprintError, SprintIndexEntry, SprintOrder, SprintStatus,
+    SprintVersion,
+};
+
 pub use live_state::{LiveEntry, LiveState, WorkStatus, FIELD_MAX_BYTES, FIELD_PREVIEW_MAX_CHARS};
 
 pub use readiness::{ReadinessPolicy, ReadinessSignal};
@@ -183,5 +189,5 @@ pub use ports::{
     IssueStoreError, LiveStateStore, MemoryError, MemoryQuery, MemoryRecall, MemoryStore, Output,
     OutputStream, PermissionStore, PreparedContext, ProcessError, ProcessSpawner, ProfileStore,
     ProjectStore, PtyError, PtyHandle, PtyPort, RemoteError, RemoteHost, RemotePath, RuntimeError,
-    ScheduledTask, Scheduler, SpawnSpec, StoreError, TemplateStore,
+    ScheduledTask, Scheduler, SpawnSpec, SprintStore, SprintStoreError, StoreError, TemplateStore,
 };