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__/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",
 ];