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/singletonAgent.test.tsx

@@ -4,11 +4,11 @@
  * - The mock agent gateway mirrors the backend guard: launching an agent already
  *   live in another cell is refused with `AGENT_ALREADY_RUNNING`, and the same
  *   node is idempotent; `listLiveAgents` reports who runs where (T5).
- * - The per-cell dropdown disables (and `onChange` rejects) an agent already live
- *   in another cell, while the agent pinned on its own cell stays selectable (T6).
+ * - The per-cell dropdown blocks an agent already visible in another cell, but
+ *   can rebind a background live session when the backend exposes its session id.
  */
 import { describe, it, expect, vi } from "vitest";
-import { render, screen, fireEvent } from "@testing-library/react";
+import { render, screen, fireEvent, waitFor } from "@testing-library/react";
 
 import type { Gateways, LiveAgent } from "@/ports";
 import {
@@ -65,7 +65,13 @@ describe("singleton agent — mock gateway guard (T5)", () => {
     );
 
     const live: LiveAgent[] = await agent.listLiveAgents("p1");
-    expect(live).toEqual([{ agentId: a.id, nodeId: "node-A" }]);
+    expect(live).toEqual([
+      {
+        agentId: a.id,
+        nodeId: "node-A",
+        sessionId: "mock-agent-session-1",
+      },
+    ]);
   });
 
   it("relaunching in the SAME node is allowed (idempotent), not refused", async () => {
@@ -104,34 +110,68 @@ describe("singleton agent — dropdown guard (T6)", () => {
     );
   }
 
-  it("disables an agent already running in another cell and rejects selecting it", async () => {
+  it("opens an agent already running elsewhere by rebinding its live session", async () => {
+    const layout = new MockLayoutGateway();
+    const agent = new MockAgentGateway();
+    const a = await agent.createAgent("p1", { name: "Busy", profileId: "p" });
+    await agent.launchAgent(
+      "p1",
+      a.id,
+      { cwd: "/x", rows: 24, cols: 80, nodeId: "old-node" },
+      noop,
+    );
+
+    const tree = await layout.loadLayout("p1");
+    const leafId = leaves(tree)[0].id;
+
+    renderGrid(layout, agent);
+
+    // The option is selectable: choosing it rebinds the running session here.
+    const option = await screen.findByRole("option", {
+      name: /Busy/,
+    });
+    expect((option as HTMLOptionElement).disabled).toBe(false);
+
+    const select = screen.getByRole("combobox") as HTMLSelectElement;
+    fireEvent.change(select, { target: { value: a.id } });
+
+    await waitFor(async () => {
+      const updated = await layout.loadLayout("p1");
+      const leaf = leaves(updated).find((l) => l.id === leafId)!;
+      expect(leaf.agent).toBe(a.id);
+      expect(leaf.session).toBe("mock-agent-session-1");
+    });
+    expect(await agent.listLiveAgents("p1")).toEqual([
+      {
+        agentId: a.id,
+        nodeId: leafId,
+        sessionId: "mock-agent-session-1",
+      },
+    ]);
+  });
+
+  it("explains the missing backend contract when a live agent has no attachable session", async () => {
     const layout = new MockLayoutGateway();
     const agent = new MockAgentGateway();
     const a = await agent.createAgent("p1", { name: "Busy", profileId: "p" });
 
-    // Mark the agent live in a DIFFERENT node than the (single) leaf we render.
     vi.spyOn(agent, "listLiveAgents").mockResolvedValue([
       { agentId: a.id, nodeId: "some-other-node" },
     ]);
 
     renderGrid(layout, agent);
 
-    // The option for the live-elsewhere agent is rendered disabled with a label.
     const option = await screen.findByRole("option", {
-      name: /Busy \(en cours ailleurs\)/,
+      name: /Busy/,
     });
-    expect((option as HTMLOptionElement).disabled).toBe(true);
+    expect((option as HTMLOptionElement).disabled).toBe(false);
 
-    // Programmatically selecting it must be rejected (agent not pinned).
     const select = screen.getByRole("combobox") as HTMLSelectElement;
-    const setCellAgentSpy = vi.spyOn(layout, "mutateLayout");
     fireEvent.change(select, { target: { value: a.id } });
 
-    // A notice is shown and no setCellAgent mutation was issued.
-    expect(await screen.findByRole("status")).toBeTruthy();
-    expect(
-      setCellAgentSpy.mock.calls.filter((c) => c[1].type === "setCellAgent"),
-    ).toHaveLength(0);
+    expect((await screen.findByRole("status")).textContent).toMatch(
+      /Session active introuvable/,
+    );
   });
 
   it("the agent pinned on THIS cell stays selectable even while live (same node)", async () => {