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>

frontend/src/features/layout/setCellAgent.test.tsx

@@ -232,15 +232,17 @@ describe("setCellAgent (agent dropdown per cell)", () => {
 
     renderGrid(layout, agent, "p1", terminal);
 
-    // Close the SECOND cell: keep the first, drop the second → its PTY dies.
+    // Close the SECOND cell: keep the first, drop the second. Closing a cell is
+    // now a detach-only view operation; it must not kill the PTY.
     await waitFor(() => {
       expect(screen.getByLabelText(`close ${droppedId}`)).toBeTruthy();
     });
     fireEvent.click(screen.getByLabelText(`close ${droppedId}`));
 
     await waitFor(() => {
-      expect(closeSpy).toHaveBeenCalledWith("dropped-session");
+      expect(screen.getAllByTestId("layout-leaf")).toHaveLength(1);
     });
+    expect(closeSpy).not.toHaveBeenCalled();
   });
 
   it("agent selector shows project agents as options", async () => {