fix(config): reject malformed cross_project_learnings at set

A typo was stored with exit 0, so the feature stayed off and the first-run prompt never returned. Reject like codex_reviews; do not coerce.

Co-authored-by: Cursor <cursoragent@cursor.com>

test/gstack-config-cross-project.test.ts

@@ -0,0 +1,67 @@
+/**
+ * #2673: gstack-config set must reject malformed cross_project_learnings.
+ *
+ * Empty get is the first-run prompt sentinel (pinned in
+ * gstack-config-defaults.test.ts). Skills only enable on the literal "true".
+ * A typo used to store verbatim and exit 0, so the feature stayed off and
+ * the prompt never returned. Unlike pair_agent / redact_prepush_hook, do
+ * not coerce to a default — that would still persist a value and still
+ * kill the sentinel. Follow codex_reviews: reject, leave existing.
+ */
+import { afterEach, beforeEach, describe, expect, test } from "bun:test";
+import { spawnSync } from "child_process";
+import * as fs from "fs";
+import * as os from "os";
+import * as path from "path";
+
+const CONFIG = path.resolve(import.meta.dir, "..", "bin", "gstack-config");
+let stateRoot: string;
+
+function cfg(args: string[]): { code: number; out: string; err: string } {
+  const r = spawnSync(CONFIG, args, {
+    encoding: "utf8",
+    env: { ...process.env, GSTACK_STATE_ROOT: stateRoot },
+  });
+  return { code: r.status ?? 0, out: r.stdout ?? "", err: r.stderr ?? "" };
+}
+
+beforeEach(() => {
+  stateRoot = fs.mkdtempSync(path.join(os.tmpdir(), "gstack-config-xproj-"));
+});
+
+afterEach(() => {
+  fs.rmSync(stateRoot, { recursive: true, force: true });
+});
+
+describe("cross_project_learnings set domain (#2673)", () => {
+  test("empty get is still the first-run sentinel", () => {
+    const r = cfg(["get", "cross_project_learnings"]);
+    expect(r.code).toBe(0);
+    expect(r.out).toBe("");
+  });
+
+  test("true and false round-trip", () => {
+    expect(cfg(["set", "cross_project_learnings", "true"]).code).toBe(0);
+    expect(cfg(["get", "cross_project_learnings"]).out).toBe("true");
+    expect(cfg(["set", "cross_project_learnings", "false"]).code).toBe(0);
+    expect(cfg(["get", "cross_project_learnings"]).out).toBe("false");
+  });
+
+  test("typo is rejected and does not write", () => {
+    const r = cfg(["set", "cross_project_learnings", "ture"]);
+    expect(r.code).toBe(1);
+    expect(r.err).toContain("not recognized");
+    expect(r.err).toContain("cross_project_learnings");
+    const got = cfg(["get", "cross_project_learnings"]);
+    expect(got.code).toBe(0);
+    expect(got.out).toBe("");
+  });
+
+  test("typo leaves an existing valid value unchanged", () => {
+    expect(cfg(["set", "cross_project_learnings", "true"]).code).toBe(0);
+    const r = cfg(["set", "cross_project_learnings", "yes"]);
+    expect(r.code).toBe(1);
+    expect(r.err).toContain("Existing value left unchanged");
+    expect(cfg(["get", "cross_project_learnings"]).out).toBe("true");
+  });
+});