feat(slash): fondation du système de commandes slash pour la CLI custom — #162 (QA verte)

Introduit le contrat unifié des commandes slash consommable par le frontend,
indépendant de la source (native, puis plugin plus tard) :

- domain: modèle pur SlashCommand / SlashCommandSource / SlashCommandAvailability,
  métadonnées UI (nom, description, disponibilité, confirmation requise) et
  validation des noms/préfixes.
- application: registry + list/filter par préfixe + plan d'exécution natif ;
  commandes natives /help et /clean (/clean = nettoyage de la vue de session
  courante ; pas de /reset distinct tant qu'aucune utilité produit ne le justifie).
- backend + app-tauri: exposition du contrat via DTO/transport + tests DTO.

Source neutre : les commandes contribuées par plugins (#165) transiteront par
le même registry sans changer le contrat frontend.

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

crates/backend/src/dto.rs

@@ -13,14 +13,15 @@ use application::{
     AgentBackgroundTaskState, AgentTicketState, AppError, AppExitWorkGuardDetail,
     AppExitWorkGuardState, AttachLiveAgentOutput, BackgroundTaskKindLabel,
     ConversationPreviewStatus, ConversationTurnWorkPreview, ConversationWorkSummary,
-    CreateProjectInput, CreateProjectOutput, GitGraphOutput, HealthInput, HealthReport, LayoutKind,
-    ListProjectsOutput, LiveSessionKind, LiveSessionSnapshot, OpenProjectOutput, ProjectWorkState,
+    CreateProjectInput, CreateProjectOutput, ExecuteSlashCommandOutput, GitGraphOutput,
+    HealthInput, HealthReport, LayoutKind, ListProjectsOutput, ListSlashCommandsOutput,
+    LiveSessionKind, LiveSessionSnapshot, OpenProjectOutput, ProjectWorkState, SlashCommandEffect,
     StopLiveAgentOutput, TicketWorkSource, TicketWorkStatus, TurnPage, TurnSource, TurnView,
 };
 use domain::ports::{ReplyProgress, ReplyProgressKind, ReplyProgressSource, ReplyProgressStage};
 use domain::{
     AgentBusyState, PageCursor, PageDirection, Project, ProjectId, ProjectSystemPermissions,
-    ResolvedAgentSystemPermissions, SystemPermissionSet, TurnRole,
+    ResolvedAgentSystemPermissions, SlashCommand, SystemPermissionSet, TurnRole,
 };
 
 pub use crate::ticket_dto::*;
@@ -764,6 +765,106 @@ impl From<HealthReport> for HealthResponseDto {
     }
 }
 
+/// Request DTO for slash-command list/filter.
+#[derive(Debug, Clone, Default, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ListSlashCommandsRequestDto {
+    /// Optional typed prefix, with or without the leading `/`.
+    #[serde(default)]
+    pub prefix: Option<String>,
+}
+
+/// UI-facing slash-command metadata DTO.
+pub type SlashCommandDto = SlashCommand;
+
+/// Slash-command list response DTO.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SlashCommandListDto {
+    /// Commands in deterministic UI order.
+    pub commands: Vec<SlashCommandDto>,
+}
+
+impl From<ListSlashCommandsOutput> for SlashCommandListDto {
+    fn from(value: ListSlashCommandsOutput) -> Self {
+        Self {
+            commands: value.commands,
+        }
+    }
+}
+
+/// Request DTO for slash-command execution.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ExecuteSlashCommandRequestDto {
+    /// Command name, with or without the leading `/`.
+    pub name: String,
+    /// Current structured chat session id, required by `/clean` and future
+    /// conversation-local commands.
+    #[serde(default)]
+    pub session_id: Option<String>,
+}
+
+/// Slash-command execution response DTO.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ExecuteSlashCommandResponseDto {
+    /// Executed command metadata.
+    pub command: SlashCommandDto,
+    /// Effect handled by the UI/adapter.
+    pub effect: SlashCommandEffectDto,
+}
+
+impl From<ExecuteSlashCommandOutput> for ExecuteSlashCommandResponseDto {
+    fn from(value: ExecuteSlashCommandOutput) -> Self {
+        Self {
+            command: value.command,
+            effect: value.effect.into(),
+        }
+    }
+}
+
+/// Slash-command execution effect DTO.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase", tag = "kind")]
+pub enum SlashCommandEffectDto {
+    /// Display command help.
+    Help {
+        /// Commands to display.
+        commands: Vec<SlashCommandDto>,
+    },
+    /// The current conversation was cleared.
+    CleanConversation {
+        /// Session id that was cleared.
+        #[serde(rename = "sessionId")]
+        session_id: String,
+        /// Number of retained chunks removed by the backend bridge.
+        #[serde(rename = "clearedChunks")]
+        cleared_chunks: usize,
+    },
+    /// Placeholder for ticket #164.
+    ProfileSwitch {
+        /// Session id targeted by the profile flow.
+        #[serde(rename = "sessionId")]
+        session_id: String,
+    },
+}
+
+impl From<SlashCommandEffect> for SlashCommandEffectDto {
+    fn from(value: SlashCommandEffect) -> Self {
+        match value {
+            SlashCommandEffect::Help { commands } => Self::Help { commands },
+            SlashCommandEffect::CleanConversation { session_id } => Self::CleanConversation {
+                session_id: session_id.to_string(),
+                cleared_chunks: 0,
+            },
+            SlashCommandEffect::ProfileSwitch { session_id } => Self::ProfileSwitch {
+                session_id: session_id.to_string(),
+            },
+        }
+    }
+}
+
 /// Error DTO returned to the frontend in the `Err` arm of every command.
 ///
 /// `code` is a stable machine-readable string (see [`AppError::code`]); the