feat(memory): config embedders (LOT C2) + suggestion contextuelle (LOT C3) + contexte projet partagé

- LOT C2 (§14.5.3) : use cases de configuration des embedders déclaratifs
  (List/Save/Delete + DescribeEmbedderEngines : modèles ONNX recommandés,
  environnement local détecté, stratégies compilées). UI EmbedderSettings.
- LOT C3 (§14.5.5) : suggestion contextuelle best-effort à l'activation quand la
  mémoire dépasse le budget de recall sans embedder configuré (event
  EmbedderSuggested, anti-spam 1×/session, « ne plus demander »).
- Contexte projet partagé .ideai/CONTEXT.md (model-agnostic) injecté à tous les
  agents/profils au lancement, avant la persona. UI ProjectContextPanel.

Tests : backend workspace vert (0 échec) ; frontend 306/306.

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

crates/application/src/layout/usecases.rs

@@ -8,7 +8,10 @@
 use std::sync::Arc;
 
 use domain::ports::{EventBus, FileSystem, ProjectStore};
-use domain::{AgentId, Direction, DomainEvent, LayoutError, LayoutId, LayoutTree, LeafCell, NodeId, ProjectId, SessionId};
+use domain::{
+    AgentId, Direction, DomainEvent, LayoutError, LayoutId, LayoutTree, LeafCell, NodeId,
+    ProjectId, SessionId,
+};
 
 use crate::error::AppError;
 
@@ -69,6 +72,14 @@ pub enum LayoutOperation {
         /// Session to host, or `None` to clear.
         session: Option<SessionId>,
     },
+    /// Attach an existing live/background session to a leaf, clearing any
+    /// previous visible host of the same session.
+    AttachSession {
+        /// The hosting leaf.
+        target: NodeId,
+        /// Session to make visible in this leaf.
+        session: SessionId,
+    },
     /// Attach or detach an agent to/from a leaf (per-cell agent, feature #3).
     SetCellAgent {
         /// The hosting leaf.
@@ -117,6 +128,7 @@ impl LayoutOperation {
             Self::Resize { container, weights } => tree.resize(*container, weights),
             Self::Move { from, to } => tree.move_session(*from, *to),
             Self::SetSession { target, session } => tree.set_session(*target, *session),
+            Self::AttachSession { target, session } => tree.attach_session(*target, *session),
             Self::SetCellAgent { target, agent } => tree.set_cell_agent(*target, *agent),
             Self::SetCellConversation {
                 target,