test(harness): isolate DATA_DIR so tests never write to the real DB

Root cause of fake connections in Usage (zed-live-*@example.com,
guard-*@example.com, zed "Account N", kimchi-nope): route-level tests
(zed-live-models, zed-native-auth) call createProviderConnection, which
persists to $DATA_DIR/db/data.sqlite. With DATA_DIR unset — the default
for `npx vitest run` — that resolved to the user's real ~/.9router DB,
appending test rows on every run. Both files documented "RUN WITH AN
ISOLATED DB" but never enforced it.

Add tests/setup/isolateDataDir.js (wired via vitest setupFiles) that
points DATA_DIR at a throwaway temp dir before src/lib/dataDir.js is
imported. Opt out with RUN_REAL=1 or an explicit DATA_DIR (used by the
*.real.test.js suites that read live credentials).

Verified: a full suite run now leaves the real DB byte-count unchanged;
new guard test tests/unit/test-data-dir-isolation.test.js locks it in.

tests/unit/test-data-dir-isolation.test.js

@@ -0,0 +1,27 @@
+// Guard: the test harness must never write into the user's real DB.
+//
+// Root cause this locks down: route-level tests (zed-live-models,
+// zed-native-auth) call createProviderConnection, which persists to
+// $DATA_DIR/db/data.sqlite. With no DATA_DIR set, that resolved to ~/.9router —
+// polluting the live DB with "zed-live-*@example.com", "guard-*@example.com"
+// and "Account N" rows on every `npx vitest run`.
+//
+// tests/setup/isolateDataDir.js redirects DATA_DIR to a temp dir unless the
+// caller opts out via RUN_REAL=1 or an explicit DATA_DIR.
+import { describe, it, expect } from "vitest";
+import os from "node:os";
+import path from "node:path";
+
+// When the caller opts into the real DB (RUN_REAL=1) or supplies DATA_DIR,
+// isolation is intentionally disabled — this guard only applies to the default.
+const ISOLATED = !process.env.RUN_REAL && !process.env.EXPECT_REAL_DATA_DIR;
+
+describe.skipIf(!ISOLATED)("test DATA_DIR isolation", () => {
+  it("points DATA_DIR at a temp dir, not ~/.9router", () => {
+    const dir = process.env.DATA_DIR;
+    expect(dir).toBeTruthy();
+    const home = path.join(os.homedir(), ".9router");
+    expect(path.resolve(dir)).not.toBe(path.resolve(home));
+    expect(dir.startsWith(os.tmpdir())).toBe(true);
+  });
+});