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/projects/projects.test.tsx

@@ -181,6 +181,13 @@ describe("ProjectsView (with MockProjectGateway)", () => {
     );
     expect(screen.queryByLabelText("project name")).toBeNull();
 
+    // Switch to Context tab — shared project context editor should be visible.
+    fireEvent.click(screen.getByRole("button", { name: "Context" }));
+    await waitFor(() =>
+      expect(screen.getByLabelText("project context")).toBeTruthy(),
+    );
+    expect(screen.queryByLabelText("agent name")).toBeNull();
+
     // Switch to Templates tab — the "New template" button should be visible.
     fireEvent.click(screen.getByRole("button", { name: "Templates" }));
     await waitFor(() =>
@@ -199,6 +206,31 @@ describe("ProjectsView (with MockProjectGateway)", () => {
     expect(screen.getByLabelText("project name")).toBeTruthy();
   });
 
+  it("edits the shared project context stored under .ideai", async () => {
+    const project = new MockProjectGateway();
+    const created = await project.createProject("alpha", "/p/a");
+    await project.updateProjectContext(created.id, "# Existing context");
+    renderView(project);
+
+    await screen.findByText("/p/a");
+    fireEvent.click(screen.getByRole("button", { name: "Open" }));
+    fireEvent.click(screen.getByRole("button", { name: "Context" }));
+
+    const editor = (await screen.findByLabelText(
+      "project context",
+    )) as HTMLTextAreaElement;
+    expect(editor.value).toBe("# Existing context");
+
+    fireEvent.change(editor, { target: { value: "# Shared\n\nUse pnpm." } });
+    fireEvent.click(screen.getByRole("button", { name: "Save" }));
+
+    await waitFor(async () => {
+      await expect(project.readProjectContext(created.id)).resolves.toBe(
+        "# Shared\n\nUse pnpm.",
+      );
+    });
+  });
+
   it("Browse… button calls pickFolder and fills the root field with the returned path", async () => {
     const system = new MockSystemGateway();
     renderView(new MockProjectGateway(), system);