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/ports.rs

@@ -30,6 +30,9 @@ use thiserror::Error;
 use crate::agent::AgentManifest;
 use crate::events::DomainEvent;
 use crate::ids::{AgentId, NodeId, ScheduleId, SessionId};
+use crate::issue::{
+    Issue, IssueCarnet, IssueIndexEntry, IssueListFilter, IssueNumber, IssueRef, IssueVersion,
+};
 use crate::markdown::MarkdownDoc;
 use crate::memory::{Memory, MemoryIndexEntry, MemoryLink, MemorySlug};
 use crate::permission::ProjectPermissions;
@@ -461,6 +464,28 @@ pub enum GitError {
     Operation(String),
 }
 
+/// Errors from the issue store.
+#[derive(Debug, Clone, PartialEq, Eq, Error)]
+pub enum IssueStoreError {
+    /// The requested issue does not exist.
+    #[error("issue not found")]
+    NotFound,
+    /// Optimistic concurrency conflict.
+    #[error("issue version conflict: expected {expected}, actual {actual}")]
+    VersionConflict {
+        /// Expected version.
+        expected: IssueVersion,
+        /// Actual persisted version.
+        actual: IssueVersion,
+    },
+    /// The issue payload is invalid.
+    #[error("issue invalid: {0}")]
+    Invalid(String),
+    /// Store I/O or serialization failed.
+    #[error("issue store failed: {0}")]
+    Store(String),
+}
+
 // ---------------------------------------------------------------------------
 // Git port support types
 // ---------------------------------------------------------------------------
@@ -1191,6 +1216,84 @@ pub trait PermissionStore: Send + Sync {
     ) -> Result<(), StoreError>;
 }
 
+/// Allocates monotonic per-project issue numbers.
+#[async_trait]
+pub trait IssueNumberAllocator: Send + Sync {
+    /// Allocates the next number for `root`.
+    ///
+    /// Implementations must never reuse an allocated number, even if a later
+    /// issue creation step fails.
+    ///
+    /// # Errors
+    /// [`IssueStoreError`] on persistence/lock failure.
+    async fn allocate_next(&self, root: &ProjectPath) -> Result<IssueNumber, IssueStoreError>;
+}
+
+/// Persistence port for project-scoped issues.
+#[async_trait]
+pub trait IssueStore: Send + Sync {
+    /// Creates a new issue.
+    ///
+    /// # Errors
+    /// [`IssueStoreError`] when the issue already exists or cannot be stored.
+    async fn create(&self, root: &ProjectPath, issue: &Issue) -> Result<(), IssueStoreError>;
+
+    /// Reads an issue by `#N` reference.
+    ///
+    /// # Errors
+    /// [`IssueStoreError::NotFound`] when absent.
+    async fn get_by_ref(
+        &self,
+        root: &ProjectPath,
+        issue_ref: IssueRef,
+    ) -> Result<Issue, IssueStoreError>;
+
+    /// Lists issue index entries matching `filter`.
+    ///
+    /// # Errors
+    /// [`IssueStoreError`] on persistence failure.
+    async fn list(
+        &self,
+        root: &ProjectPath,
+        filter: IssueListFilter,
+    ) -> Result<Vec<IssueIndexEntry>, IssueStoreError>;
+
+    /// Replaces an issue after checking its expected version.
+    ///
+    /// # Errors
+    /// [`IssueStoreError::VersionConflict`] on optimistic-concurrency conflict.
+    async fn update(
+        &self,
+        root: &ProjectPath,
+        issue: &Issue,
+        expected_version: IssueVersion,
+    ) -> Result<(), IssueStoreError>;
+
+    /// Reads only the issue carnet projection.
+    ///
+    /// # Errors
+    /// [`IssueStoreError::NotFound`] when absent.
+    async fn read_carnet(
+        &self,
+        root: &ProjectPath,
+        issue_ref: IssueRef,
+    ) -> Result<IssueCarnet, IssueStoreError>;
+
+    /// Replaces an issue carnet and increments the issue version.
+    ///
+    /// # Errors
+    /// [`IssueStoreError::VersionConflict`] on optimistic-concurrency conflict.
+    async fn write_carnet(
+        &self,
+        root: &ProjectPath,
+        issue_ref: IssueRef,
+        carnet: MarkdownDoc,
+        actor: crate::issue::IssueActor,
+        now_ms: u64,
+        expected_version: IssueVersion,
+    ) -> Result<IssueCarnet, IssueStoreError>;
+}
+
 /// Git operations for a project. Named `GitPort` to avoid clashing with the
 /// [`crate::git::GitRepository`] *entity* (state image).
 #[async_trait]