feat(wave2): add @mosaic/openclaw-context plugin to monorepo

- OpenClaw → OpenBrain context engine plugin migrated from mosaic/openclaw-openbrain-context
- Depends on @mosaic/types workspace:*
- Typecheck passing

plugins/openclaw-context/tests/smoke.test.ts

@@ -0,0 +1,9 @@
+import { describe, expect, it } from "vitest";
+
+import { OPENBRAIN_CONTEXT_ENGINE_ID } from "../src/index.js";
+
+describe("project scaffold", () => {
+  it("exports openbrain context engine id", () => {
+    expect(OPENBRAIN_CONTEXT_ENGINE_ID).toBe("openbrain");
+  });
+});