feat(tickets): checkpoint backend V1 du système de tickets (T1-T5)

Checkpoint de travail — QA formelle encore à venir (après reset Codex).
`cargo build` workspace OK, tests ticket/issue verts (domaine, store,
use cases, app-tauri 47/51). Le domaine s'appelle `Issue`, exposé
« ticket » côté MCP/UI.

- T1 domaine : entité Issue (statut, priorité, liens, carnet),
  events, ids, ports.
- T2 infra : store FS Markdown + allocator de références #N.
- T3 application : use cases Issue (create/read/list/update/status/
  priority/carnet/link/unlink/assign) + erreurs dédiées.
- T4 surface MCP : 10 outils publics idea_ticket_* (23 outils au
  total), mappés vers les use cases Issue.
- T5 app-tauri : commandes Tauri ticket_* + DTOs d'events Issue +
  câblage state.rs.

Dette de test PRÉ-EXISTANTE héritée de develop (4 tests app-tauri
mcp_e2e_loopback / mcp_serve_peer rouges) hors périmètre tickets,
non traitée ici.

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

crates/domain/src/events.rs

@@ -1,7 +1,8 @@
 //! Domain events published on the [`crate::ports::EventBus`] and relayed to the
 //! presentation layer (ARCHITECTURE §3.2).
 
-use crate::ids::{AgentId, ProfileId, ProjectId, SessionId, SkillId, TemplateId};
+use crate::ids::{AgentId, IssueId, ProfileId, ProjectId, SessionId, SkillId, TemplateId};
+use crate::issue::{IssueLinkKind, IssuePriority, IssueRef, IssueStatus, IssueVersion};
 use crate::mailbox::TicketId;
 use crate::memory::MemorySlug;
 use crate::template::TemplateVersion;
@@ -168,6 +169,85 @@ pub enum DomainEvent {
         /// The project whose index was rebuilt.
         project_id: ProjectId,
     },
+    /// An issue was created.
+    IssueCreated {
+        /// Stable issue id.
+        issue_id: IssueId,
+        /// Human-friendly issue reference.
+        issue_ref: IssueRef,
+    },
+    /// An issue was updated.
+    IssueUpdated {
+        /// Human-friendly issue reference.
+        issue_ref: IssueRef,
+        /// New optimistic version.
+        version: IssueVersion,
+    },
+    /// An issue status changed.
+    IssueStatusChanged {
+        /// Human-friendly issue reference.
+        issue_ref: IssueRef,
+        /// New status.
+        status: IssueStatus,
+        /// New optimistic version.
+        version: IssueVersion,
+    },
+    /// An issue priority changed.
+    IssuePriorityChanged {
+        /// Human-friendly issue reference.
+        issue_ref: IssueRef,
+        /// New priority.
+        priority: IssuePriority,
+        /// New optimistic version.
+        version: IssueVersion,
+    },
+    /// An issue carnet was updated.
+    IssueCarnetUpdated {
+        /// Human-friendly issue reference.
+        issue_ref: IssueRef,
+        /// New optimistic version.
+        version: IssueVersion,
+    },
+    /// Two issues were linked.
+    IssueLinked {
+        /// Source issue.
+        issue_ref: IssueRef,
+        /// Target issue.
+        target: IssueRef,
+        /// Link kind.
+        kind: IssueLinkKind,
+        /// New optimistic version.
+        version: IssueVersion,
+    },
+    /// Two issues were unlinked.
+    IssueUnlinked {
+        /// Source issue.
+        issue_ref: IssueRef,
+        /// Target issue.
+        target: IssueRef,
+        /// Link kind.
+        kind: IssueLinkKind,
+        /// New optimistic version.
+        version: IssueVersion,
+    },
+    /// An agent was assigned to an issue.
+    IssueAgentAssigned {
+        /// Human-friendly issue reference.
+        issue_ref: IssueRef,
+        /// Assigned agent.
+        agent_id: AgentId,
+        /// New optimistic version.
+        version: IssueVersion,
+    },
+    /// An agent was unassigned from an issue.
+    IssueAgentUnassigned {
+        /// Human-friendly issue reference.
+        issue_ref: IssueRef,
+        /// Unassigned agent.
+        agent_id: AgentId,
+        /// New optimistic version.
+        version: IssueVersion,
+    },
     /// A project's memory grew past the recall budget while no embedder is
     /// configured (strategy `none`): the first moment a semantic embedder would
     /// help (ARCHITECTURE §14.5.5, LOT C3). Published **at most once per session per