fix(test): use mkdtemp for unique temp dir in plan-manager contract tests

Static path 'plan-manager-contract-tests' caused EINVAL races when
concurrent test workers cleaned up the shared directory while another
test was still writing to it. Replace with mkdtemp per setup() call so
each test run gets an isolated directory.

Author: mrsimpson

packages/core/test/unit/contracts/plan-manager-contract.test.ts

@@ -8,6 +8,7 @@
 import { describe, it, expect } from 'vitest';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
+import { mkdtemp } from 'node:fs/promises';
 import {
   BaseInterfaceContract,
   ValidationHelpers,
@@ -55,10 +56,12 @@ const mockStateMachine = {
   },
 };
 
-// Create test paths using temp directory
-const testDir = join(tmpdir(), 'plan-manager-contract-tests');
-const testPlanPath = join(testDir, 'plan.md');
-const testProjectPath = join(testDir, 'project');
+// Paths are resolved lazily in setup() so each test run gets a unique directory.
+// Using a static path caused flakiness when cleanup in one test deleted the
+// directory while another concurrent test was still writing to it.
+let testDir = join(tmpdir(), 'plan-manager-contract-tests');
+let testPlanPath = join(testDir, 'plan.md');
+let testProjectPath = join(testDir, 'project');
 
 /**
  * Plan Manager Contract Test Suite
@@ -315,9 +318,10 @@ describe('IPlanManager Interface Contract', () => {
       return new PlanManager();
     },
     setup: async (instance: IPlanManager) => {
-      // Ensure test directory exists
-      const { mkdir } = await import('node:fs/promises');
-      await mkdir(testDir, { recursive: true });
+      // Create a unique temp directory per test run to avoid concurrent-cleanup races
+      testDir = await mkdtemp(join(tmpdir(), 'plan-manager-contract-'));
+      testPlanPath = join(testDir, 'plan.md');
+      testProjectPath = join(testDir, 'project');
 
       // Set up state machine for PlanManager
       (