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/application/tests/snapshot_running_agents.rs

@@ -222,6 +222,7 @@ fn was_running(fs: &FakeFs, node: NodeId) -> Option<bool> {
         match node {
             LayoutNode::Leaf(l) if l.id == target => Some(l.agent_was_running),
             LayoutNode::Leaf(_) => None,
+            LayoutNode::CustomPluginLayout(_) => None,
             LayoutNode::Split(s) => s.children.iter().find_map(|c| find(&c.node, target)),
             LayoutNode::Grid(g) => g.cells.iter().find_map(|c| find(&c.node, target)),
         }