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__/check-test-isolation.test.mjs

@@ -3,7 +3,7 @@ import assert from "node:assert/strict";
 import { mkdtempSync, mkdirSync, writeFileSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import path from "node:path";
-import { spawnSync } from "node:child_process";
+import { spawnSync, spawn } from "node:child_process";
 
 const scriptPath = path.resolve("scripts/check-test-isolation.mjs");
 
@@ -82,3 +82,36 @@ test("fails when protected .fusion existence changes after baseline", () => {
     assert.match(after.stderr, /protected live \.fusion data changed/i);
   });
 });
+
+test("passes when HOME .fusion is externally active during baseline and check", () => {
+  withFixture(({ cwd, home }) => {
+    const churnScript = `
+      const fs = require("node:fs");
+      const path = require("node:path");
+      const home = process.argv[2];
+      const target = path.join(home, ".fusion", "external-churn.txt");
+      let n = 0;
+      const timer = setInterval(() => {
+        fs.writeFileSync(target, String(n++));
+      }, 120);
+      setTimeout(() => {
+        clearInterval(timer);
+        process.exit(0);
+      }, 3500);
+    `;
+
+    const churn = spawn(process.execPath, ["-e", churnScript, home], {
+      cwd,
+      env: { ...process.env, HOME: home, USERPROFILE: home },
+      stdio: "ignore",
+    });
+
+    const before = runScript(["--before"], { cwd, home });
+    assert.equal(before.status, 0);
+    const after = runScript([], { cwd, home });
+    assert.equal(after.status, 0);
+
+    churn.kill("SIGTERM");
+    rmSync(path.join(home, ".fusion", "external-churn.txt"), { force: true });
+  });
+});

scripts/__tests__/plugin-authoring-docs.test.mjs

@@ -26,7 +26,7 @@ const expectedSections = [
   "Example Plugins",
   "Registering Skills",
   "Registering Workflow Steps",
-  "Plugin Prompt Contributions",
+  "Contributing Prompt Modifications",
   "Plugin Binary Setup Hooks",
 ];
 

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 });
+});