feat(plugins): seam slash-commands contribuées par plugin via callback — #165 (QA verte)

Permet à un plugin de contribuer des commandes slash adossées à une callback,
transitant par le registry/contrat unifié (#162) :

- sdk/IdeaSDK: manifeste contributes.slashCommands (déclaratif) — pointer bumpé.
- domain: source Plugin étendue + effet PluginCallback (la commande ne
  s'exécute pas directement : elle renvoie une référence à la callback du plugin).
- application: registration des commandes plugin dans le registry + exécution
  renvoyant l'effet PluginCallback ; métadonnées UI suffisantes pour l'autocomplete.
- backend + app-tauri: DTO + commandes Tauri listant/invoquant les commandes plugin.
- frontend (contrat): types adapters/domain/ports/mock pour pluginCallback et
  l'invocation avec arguments.

L'exécution effective de la callback côté runtime UI est livrée par #166.

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

crates/app-tauri/src/commands.rs

@@ -11,6 +11,7 @@ use tauri::ipc::Channel;
 use tauri::{AppHandle, Emitter, Manager, State, WebviewUrl, WebviewWindowBuilder, WindowEvent};
 
 use crate::dto::DismissEmbedderSuggestionRequestDto;
+use application::plugin_slash_commands_from_runtime_catalog;
 use application::{
     AppError, AssignSkillToAgentInput, AttachLiveAgentInput, ChangeAgentProfileInput,
     CloseProjectInput, CreateAgentInput, CreateLayoutInput, CreateMemoryInput, CreateSkillInput,
@@ -110,17 +111,27 @@ pub fn health(
 
 /// `list_slash_commands` — list/filter the unified slash-command registry.
 #[tauri::command]
-pub fn list_slash_commands(
+pub async fn list_slash_commands(
     request: Option<crate::dto::ListSlashCommandsRequestDto>,
     state: State<'_, AppState>,
-) -> SlashCommandListDto {
+) -> Result<SlashCommandListDto, ErrorDto> {
     let request = request.unwrap_or_default();
-    state
+    let catalog = state
+        .list_plugin_runtime_contributions
+        .execute()
+        .await
+        .map_err(ErrorDto::from)?;
+    let plugin_commands =
+        plugin_slash_commands_from_runtime_catalog(&catalog).map_err(ErrorDto::from)?;
+    Ok(state
         .list_slash_commands
-        .execute(ListSlashCommandsInput {
-            prefix: request.prefix,
-        })
-        .into()
+        .execute_with_plugins(
+            ListSlashCommandsInput {
+                prefix: request.prefix,
+            },
+            plugin_commands,
+        )
+        .into())
 }
 
 /// `execute_slash_command` — execute a selected slash command.
@@ -129,7 +140,7 @@ pub fn list_slash_commands(
 /// Returns an [`ErrorDto`] (`INVALID` for unavailable commands/missing context,
 /// `NOT_FOUND` for unknown commands or closed sessions).
 #[tauri::command]
-pub fn execute_slash_command(
+pub async fn execute_slash_command(
     request: ExecuteSlashCommandRequestDto,
     state: State<'_, AppState>,
 ) -> Result<ExecuteSlashCommandResponseDto, ErrorDto> {
@@ -138,12 +149,23 @@ pub fn execute_slash_command(
         .as_deref()
         .map(parse_session_id)
         .transpose()?;
+    let catalog = state
+        .list_plugin_runtime_contributions
+        .execute()
+        .await
+        .map_err(ErrorDto::from)?;
+    let plugin_commands =
+        plugin_slash_commands_from_runtime_catalog(&catalog).map_err(ErrorDto::from)?;
     let output = state
         .execute_slash_command
-        .execute(ExecuteSlashCommandInput {
-            name: request.name,
-            session_id,
-        })
+        .execute_with_plugins(
+            ExecuteSlashCommandInput {
+                name: request.name,
+                session_id,
+                arguments: request.arguments,
+            },
+            plugin_commands,
+        )
         .map_err(ErrorDto::from)?;
 
     let effect = match output.effect {
@@ -163,6 +185,17 @@ pub fn execute_slash_command(
         SlashCommandEffect::ProfileSwitch { session_id } => SlashCommandEffectDto::ProfileSwitch {
             session_id: session_id.to_string(),
         },
+        SlashCommandEffect::PluginCallback {
+            plugin_id,
+            command_id,
+            session_id,
+            arguments,
+        } => SlashCommandEffectDto::PluginCallback {
+            plugin_id,
+            command_id,
+            session_id: session_id.map(|id| id.to_string()),
+            arguments,
+        },
     };
 
     Ok(ExecuteSlashCommandResponseDto {