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/lib.rs

@@ -39,6 +39,7 @@ pub mod fileguard;
 pub mod git;
 pub mod ids;
 pub mod input;
+pub mod issue;
 pub mod layout;
 pub mod live_state;
 pub mod mailbox;
@@ -67,8 +68,8 @@ mod validation;
 pub use error::DomainError;
 
 pub use ids::{
-    AgentId, LayoutId, NodeId, ProfileId, ProjectId, ScheduleId, SessionId, SkillId, TabId,
-    TemplateId, WindowId,
+    AgentId, IssueId, LayoutId, NodeId, ProfileId, ProjectId, ScheduleId, SessionId, SkillId,
+    TabId, TemplateId, WindowId,
 };
 
 pub use project::{Project, ProjectPath};
@@ -96,6 +97,12 @@ pub use conversation::{
 
 pub use input::{AgentBusyState, AgentLiveness, InputMediator, InputSource};
 
+pub use issue::{
+    AgentIssueRef, AgentIssueRole, Issue, IssueActor, IssueCarnet, IssueError, IssueIndexEntry,
+    IssueLink, IssueLinkKind, IssueListFilter, IssueNumber, IssuePriority, IssueRef, IssueStatus,
+    IssueVersion,
+};
+
 pub use live_state::{LiveEntry, LiveState, WorkStatus, FIELD_MAX_BYTES, FIELD_PREVIEW_MAX_CHARS};
 
 pub use readiness::{ReadinessPolicy, ReadinessSignal};
@@ -158,8 +165,9 @@ pub use ports::{
     EmbedderEnvInspector, EmbedderEnvReport, EmbedderError, EmbedderProfileStore,
     EmbedderPromptDismissal, EmbedderPromptStore, EventBus, EventStream, ExitStatus, FileSystem,
     FsError, GitCommitInfo, GitError, GitFileStatus, GitPort, GraphCommit, IdGenerator,
-    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,
+    IssueNumberAllocator, IssueStore, 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,
 };