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/application/src/error.rs

@@ -9,8 +9,8 @@ use domain::ports::{
     AgentSessionError, EmbedderError, FsError, GitError, MemoryError, ProcessError, PtyError,
     RemoteError, RuntimeError, StoreError,
 };
-use domain::IssueStoreError;
 use domain::{AgentId, NodeId};
+use domain::{IssueStoreError, SprintStoreError};
 
 /// Errors surfaced by application use cases.
 ///
@@ -140,6 +140,17 @@ impl From<IssueStoreError> for AppError {
     }
 }
 
+impl From<SprintStoreError> for AppError {
+    fn from(e: SprintStoreError) -> Self {
+        match e {
+            SprintStoreError::NotFound => Self::NotFound("sprint".to_owned()),
+            SprintStoreError::VersionConflict { .. } => Self::Invalid(e.to_string()),
+            SprintStoreError::Invalid(message) => Self::Invalid(message),
+            SprintStoreError::Store(message) => Self::Store(message),
+        }
+    }
+}
+
 impl From<MemoryError> for AppError {
     fn from(e: MemoryError) -> Self {
         match e {