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/domain/tests/layout.rs

@@ -3,11 +3,11 @@
 
 mod helpers;
 
+use domain::ids::AgentId;
 use domain::{
     Direction, GridCell, GridContainer, LayoutError, LayoutNode, LayoutTree, LeafCell,
     SplitContainer, WeightedChild,
 };
-use domain::ids::AgentId;
 use helpers::{node, session};
 
 fn agent_id(n: u128) -> AgentId {
@@ -463,6 +463,36 @@ fn set_session_duplicate_across_leaves_rejected() {
     assert_eq!(err, LayoutError::DuplicateSession(session(100)));
 }
 
+#[test]
+fn attach_session_attaches_background_session_to_leaf() {
+    let tree = two_leaves(None, None);
+    let out = tree.attach_session(node(2), session(100)).unwrap();
+    assert_eq!(session_for(&out, node(1)), None);
+    assert_eq!(session_for(&out, node(2)), Some(session(100)));
+}
+
+#[test]
+fn attach_session_moves_visible_session_to_target_leaf() {
+    let tree = two_leaves(Some(100), None);
+    let out = tree.attach_session(node(2), session(100)).unwrap();
+    assert_eq!(session_for(&out, node(1)), None);
+    assert_eq!(session_for(&out, node(2)), Some(session(100)));
+}
+
+#[test]
+fn attach_session_same_leaf_is_idempotent() {
+    let tree = two_leaves(Some(100), None);
+    let out = tree.attach_session(node(1), session(100)).unwrap();
+    assert_eq!(out, tree);
+}
+
+#[test]
+fn attach_session_rejects_target_occupied_by_other_session() {
+    let tree = two_leaves(Some(100), Some(200));
+    let err = tree.attach_session(node(2), session(100)).unwrap_err();
+    assert_eq!(err, LayoutError::CrossContainer);
+}
+
 // ---------------------------------------------------------------------------
 // set_cell_agent (#3: per-cell agent)
 // ---------------------------------------------------------------------------
@@ -503,7 +533,9 @@ fn set_cell_agent_is_immutable_source_unchanged() {
 #[test]
 fn set_cell_agent_missing_leaf_is_node_not_found() {
     let tree = single(1, None);
-    let err = tree.set_cell_agent(node(404), Some(agent_id(1))).unwrap_err();
+    let err = tree
+        .set_cell_agent(node(404), Some(agent_id(1)))
+        .unwrap_err();
     assert_eq!(err, LayoutError::NodeNotFound(node(404)));
 }