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/memory/memory.test.tsx

@@ -18,7 +18,7 @@
 import { describe, it, expect, vi } from "vitest";
 import { render, screen, waitFor, fireEvent } from "@testing-library/react";
 
-import { MockMemoryGateway } from "@/adapters/mock";
+import { MockMemoryGateway, MockEmbedderGateway } from "@/adapters/mock";
 import type { GatewayError } from "@/domain";
 import type { Gateways } from "@/ports";
 import { DIProvider } from "@/app/di";
@@ -28,7 +28,10 @@ const PROJECT_ID = "proj-memory-test";
 
 function renderMemoryPanel(memory?: MockMemoryGateway) {
   const mem = memory ?? new MockMemoryGateway();
-  const gateways = { memory: mem } as unknown as Gateways;
+  const gateways = {
+    memory: mem,
+    embedder: new MockEmbedderGateway(),
+  } as unknown as Gateways;
   return {
     memory: mem,
     ...render(