FN-6195: isolate and quarantine flaky db-corruption test

Stop the self-healing DB corruption test from leaking into unrelated tmpdir state while quarantining the known flake.

- mock node:os tmpdir usage so the test runs inside a per-test sandbox and cleans it up after each case
- add the flaky self-healing DB corruption test to the engine default exclude list
- record the quarantine entry and failure context in scripts/lib/test-quarantine.json

Files changed:
 .../__tests__/self-healing-db-corruption.test.ts   | 22 ++++++++++++++++++++++
 packages/engine/vitest.config.ts                   |  1 +
 scripts/lib/test-quarantine.json                   |  8 +++++++-
 3 files changed, 30 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-6195

Fusion-Task-Lineage: 60f7f6ca-d14d-42b4-b47e-0644a25e64ae

packages/engine/src/__tests__/self-healing-db-corruption.test.ts

@@ -1,8 +1,18 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { EventEmitter } from "node:events";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
 
 import type { Settings, TaskStore } from "@fusion/core";
 
+const osState = vi.hoisted(() => ({ tempRoot: "" }));
+
+vi.mock("node:os", async () => {
+  const actual = await vi.importActual<typeof import("node:os")>("node:os");
+  return { ...actual, tmpdir: vi.fn(() => osState.tempRoot || actual.tmpdir()) };
+});
+
 import { SelfHealingManager } from "../self-healing.js";
 import type { NotificationService } from "../notification/notification-service.js";
 import * as notifierModule from "../notifier.js";
@@ -97,8 +107,13 @@ function stubMaintenance(manager: SelfHealingManager) {
   vi.spyOn(manager, "archiveStaleDoneTasks").mockResolvedValue(0);
 }
 
+const RM = { recursive: true, force: true, maxRetries: 5, retryDelay: 50 } as const;
+let sandboxRoot = "";
+
 describe("FN-5284: self-healing DB corruption surfacing", () => {
   beforeEach(() => {
+    sandboxRoot = mkdtempSync(join(tmpdir(), "fusion-db-corruption-sandbox-"));
+    osState.tempRoot = sandboxRoot;
     vi.useFakeTimers();
     vi.setSystemTime(new Date("2026-05-20T00:00:00.000Z"));
     vi.spyOn(notifierModule, "getActiveNotificationService").mockReturnValue(undefined);
@@ -108,6 +123,13 @@ describe("FN-5284: self-healing DB corruption surfacing", () => {
   afterEach(() => {
     vi.restoreAllMocks();
     vi.useRealTimers();
+    osState.tempRoot = "";
+    try {
+      rmSync(sandboxRoot, RM);
+    } catch {
+      // Best-effort cleanup only.
+    }
+    sandboxRoot = "";
   });
 
   it("does not dispatch or audit when the database is healthy", async () => {

packages/engine/vitest.config.ts

@@ -95,6 +95,7 @@ export default defineConfig({
           include: ["src/**/*.test.ts"],
           exclude: [
             "src/__tests__/reliability-interactions/**/*.test.ts",
+            "src/__tests__/self-healing-db-corruption.test.ts",
             // Real-git heavy files run in the engine-slow project so local
             // `pnpm test` stays snappy. CI picks them up via `test:slow`
             // / `test:all` invoked from the root `test:full` script.

scripts/lib/test-quarantine.json

@@ -1,4 +1,10 @@
 {
   "$comment": "Flaky-test quarantine ledger (deletion ratchet — see AGENTS.md 'Flaky tests: quarantine on sight' and docs/testing.md 'Quarantine ledger and the deletion ratchet'). A test observed failing without a corresponding real bug is quarantined ON SIGHT: add an entry here AND a matching one-line `exclude` entry in that package's vitest config, in the same commit. Every entry needs a non-empty `reason` (link the failing run) and a `quarantinedAt` date — the entry expires 14 days later, at which point the test file is DELETED unless someone rescues it with evidence it catches real regressions plus a root-cause fix (never appeasement). There is deliberately no loader module and no automation around this file: it is a dated record, the vitest config exclude is the mechanism, and the sweep is policy executed by whoever touches the suite.",
-  "entries": []
+  "entries": [
+    {
+      "file": "packages/engine/src/__tests__/self-healing-db-corruption.test.ts",
+      "reason": "FN-6195: observed flake when runMaintenance() scanned the real OS tmpdir and picked up stale fusion-ai-merge-* worktrees from unrelated tests, adding unexpected audit calls. Failing run: Fusion task FN-6195 agent verification log, 2026-06-10.",
+      "quarantinedAt": "2026-06-10"
+    }
+  ]
 }