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/ProjectContextPanel.tsx

@@ -0,0 +1,115 @@
+/**
+ * `ProjectContextPanel` — edits the shared project context stored in
+ * `.ideai/CONTEXT.md`.
+ *
+ * This context is injected into every agent launch before the agent persona,
+ * regardless of the selected AI profile/model.
+ */
+
+import { useEffect, useState } from "react";
+
+import type { GatewayError } from "@/domain";
+import { Button, Panel, Spinner, cn } from "@/shared";
+import { useGateways } from "@/app/di";
+
+export interface ProjectContextPanelProps {
+  /** Project whose `.ideai/CONTEXT.md` should be edited. */
+  projectId: string;
+}
+
+function describe(e: unknown): string {
+  if (e && typeof e === "object" && "message" in e) {
+    return String((e as GatewayError).message);
+  }
+  return String(e);
+}
+
+export function ProjectContextPanel({ projectId }: ProjectContextPanelProps) {
+  const { project } = useGateways();
+  const [content, setContent] = useState("");
+  const [savedContent, setSavedContent] = useState("");
+  const [busy, setBusy] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  useEffect(() => {
+    let cancelled = false;
+    setBusy(true);
+    setError(null);
+    project
+      .readProjectContext(projectId)
+      .then((text) => {
+        if (!cancelled) {
+          setContent(text);
+          setSavedContent(text);
+        }
+      })
+      .catch((e) => {
+        if (!cancelled) setError(describe(e));
+      })
+      .finally(() => {
+        if (!cancelled) setBusy(false);
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [project, projectId]);
+
+  async function save() {
+    setBusy(true);
+    setError(null);
+    try {
+      await project.updateProjectContext(projectId, content);
+      setSavedContent(content);
+    } catch (e) {
+      setError(describe(e));
+    } finally {
+      setBusy(false);
+    }
+  }
+
+  const dirty = content !== savedContent;
+
+  return (
+    <Panel title="Project Context" className="flex flex-col gap-0">
+      {error && (
+        <p
+          role="alert"
+          className="mx-4 mt-3 rounded-md border border-danger/40 bg-danger/10 px-3 py-2 text-sm text-danger"
+        >
+          {error}
+        </p>
+      )}
+
+      <div className="border-b border-border px-4 py-2">
+        <code className="text-xs text-muted">.ideai/CONTEXT.md</code>
+      </div>
+
+      <div className="flex flex-col gap-3 p-4">
+        <textarea
+          aria-label="project context"
+          value={content}
+          onChange={(e) => setContent(e.target.value)}
+          rows={18}
+          className={cn(
+            "w-full rounded-md bg-raised px-3 py-2 text-sm text-content",
+            "border border-border outline-none transition-colors",
+            "focus:border-primary placeholder:text-faint",
+            "disabled:cursor-not-allowed disabled:opacity-50 resize-y font-mono",
+          )}
+          disabled={busy}
+        />
+
+        <div className="flex items-center justify-end gap-2">
+          {busy && <Spinner size={14} />}
+          <Button
+            variant="primary"
+            disabled={busy || !dirty}
+            onClick={() => void save()}
+          >
+            Save
+          </Button>
+        </div>
+      </div>
+    </Panel>
+  );
+}