feat(backend): système de plugins — domaine, application, infrastructure (#43)

Lots B1-B4 : modèle de domaine des plugins (manifeste, capacités menus/layouts/MCP),
port et registre applicatif, chargement/validation en infrastructure, exposition DTO
et commandes Tauri. Tests cargo verts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

crates/domain/src/events.rs

@@ -10,6 +10,7 @@ use crate::ids::{
 use crate::issue::{IssueLinkKind, IssuePriority, IssueRef, IssueStatus, IssueVersion};
 use crate::mailbox::TicketId;
 use crate::memory::MemorySlug;
+use crate::plugin::{PluginId, PluginMcpServerId, PluginVersion};
 use crate::sprint::{SprintOrder, SprintVersion};
 use crate::template::TemplateVersion;
 
@@ -41,6 +42,53 @@ pub enum DomainEvent {
         /// The new project.
         project_id: ProjectId,
     },
+    /// A plugin was installed.
+    PluginInstalled {
+        /// Plugin id.
+        plugin_id: PluginId,
+        /// Installed version.
+        version: PluginVersion,
+    },
+    /// A plugin was enabled.
+    PluginEnabled {
+        /// Plugin id.
+        plugin_id: PluginId,
+    },
+    /// A plugin was disabled.
+    PluginDisabled {
+        /// Plugin id.
+        plugin_id: PluginId,
+        /// Whether a restart is needed for full JS purge.
+        restart_required: bool,
+    },
+    /// A plugin was uninstalled.
+    PluginUninstalled {
+        /// Plugin id.
+        plugin_id: PluginId,
+        /// Whether a restart is needed for full JS purge.
+        restart_required: bool,
+    },
+    /// A plugin failed to load.
+    PluginLoadFailed {
+        /// Plugin id.
+        plugin_id: PluginId,
+        /// Failure reason.
+        reason: String,
+    },
+    /// A plugin MCP server started.
+    PluginMcpServerStarted {
+        /// Plugin id.
+        plugin_id: PluginId,
+        /// Server id.
+        server_id: PluginMcpServerId,
+    },
+    /// A plugin MCP server stopped.
+    PluginMcpServerStopped {
+        /// Plugin id.
+        plugin_id: PluginId,
+        /// Server id.
+        server_id: PluginMcpServerId,
+    },
     /// An agent was launched in a terminal.
     AgentLaunched {
         /// The agent.