feat(sprints): surface frontend du modèle de sprints

Ticket #10 — volet frontend des sprints.

- Domaine et ports frontend étendus au modèle de sprints (domain/index.ts,
  ports/index.ts).
- Adaptateurs : ticket.ts et mock/index.ts câblent les opérations sprints.
- UI tickets : useTickets + TicketsPanel exposent le rattachement/gestion des
  sprints, avec tests Vitest associés.

Typecheck / vitest / build verts.

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

frontend/src/ports/index.ts

@@ -39,6 +39,7 @@ import type {
   ResumableAgent,
   Skill,
   SkillScope,
+  Sprint,
   Template,
   TerminalSession,
   Ticket,
@@ -794,6 +795,18 @@ export interface TicketGateway {
     assigned: boolean,
     expectedVersion: number,
   ): Promise<Ticket>;
+  /** Lists the project's sprints, ordered by `order` (ticket #10). */
+  listSprints(projectId: string): Promise<Sprint[]>;
+  /**
+   * Sets (or clears with `sprintId === null`) a ticket's sprint membership
+   * (ticket #10). Optimistic concurrency: `expectedVersion` must match.
+   */
+  setTicketSprint(
+    projectId: string,
+    ref: string,
+    sprintId: string | null,
+    expectedVersion: number,
+  ): Promise<Ticket>;
 }
 
 /**