fix(FN-3610): isolate test home for changed-package test runs

- Create a disposable HOME/USERPROFILE env for scripts/test-changed.mjs execution
- Run isolation guard checks with the same isolated env, including cache-hit no-op paths
- Clean up temp HOME after test execution to avoid residue
- Update test coverage and contributing docs for the shared isolation behavior

Fusion-Task-Id: FN-3610

scripts/__tests__/test-changed.test.mjs

@@ -19,6 +19,7 @@ import {
   cacheFilePath,
   shouldRunIsolationGuard,
   defaultTestWorkerBudget,
+  createIsolatedHomeEnv,
 } from "../test-changed.mjs";
 
 import { mkdirSync, writeFileSync, mkdtempSync, rmSync } from "node:fs";
@@ -625,3 +626,16 @@ test("defaultTestWorkerBudget: uses CPU-aware defaults and clamps concurrency",
   assert.ok(budget.totalWorkers <= 12);
   assert.equal(budget.concurrency, budget.totalWorkers);
 });
+
+test("createIsolatedHomeEnv: returns temp HOME/USERPROFILE pair without mutating input", () => {
+  const baseEnv = { PATH: process.env.PATH || "" };
+  const { env, isolatedHome } = createIsolatedHomeEnv(baseEnv);
+
+  assert.equal(env.HOME, isolatedHome);
+  assert.equal(env.USERPROFILE, isolatedHome);
+  assert.equal(baseEnv.HOME, undefined);
+  assert.equal(baseEnv.USERPROFILE, undefined);
+  assert.match(isolatedHome, /fusion-test-home-root-/);
+
+  rmSync(isolatedHome, { recursive: true, force: true });
+});