fix(#3156): sandbox HOME on raw installer spawns — the one leak no scrub reaches

The strict live-config guard this PR ships went red on CI: the suite creates
$HOME/.gsd/defaults.json. Diagnosed rather than suppressed, because the guard
is right — this is #2665's class arriving through the one door the scrub set is
structurally unable to close.

bin/install.js writeNonClaudeDefaults() (#2834) writes
path.join(os.homedir(), '.gsd', 'defaults.json') for every non-Claude runtime.
os.homedir() consults NO GSD variable, so:

  - no entry in CONFIG_LOCATION_ENV_KEYS can reach it, however the set is
    derived; and
  - blanking GSD_HOME does not reach it either -- a blank GSD_HOME falls back
    to exactly that homedir().

Only a sandboxed HOME contains it, and HOME is deliberately excluded from
TEST_ENV_BASE because blanking it would break far more than it fixed. So the
containment belongs per-spawn, which is the discipline the suite already
applies by hand -- install-minimal-hooks.test.cjs:576 carries a comment
naming this exact hazard for the --codex spawn, while the parameterized
--${runtime} spawn 130 lines below it does not. Instance fixed, class open.

Rather than add a fifth hand-synced env shape to a PR whose subject is that
hand-synced copies drift, this adds ONE export -- installSpawnEnv() in
tests/helpers.cjs -- and routes every raw installer spawn through it, including
the shared tests/helpers/install-shared.cjs installerEnv(), which every install
suite already consumes. Callers passing an explicit { HOME, USERPROFILE } are
unaffected: overrides spread last.

Census (measured, not reasoned): of the 119 test files that spawn bin/install.js,
exactly four wrote into a sandboxed $HOME before this commit -- install,
copilot-install, install-minimal-hooks, opencode-plugin-adapter -- and zero do
after. Only install.test.cjs sits in CI's targeted lane, which is why ubuntu
went red on one file while the macOS full lanes went red on four.

Attribution: the leak reproduces unchanged at upstream/next itself, so the
defect is base-owned and pre-existing; only the detector is new. The guard
found a real leak on next within one run.

No new failures: the surviving names under a sandboxed HOME
(folded:enh-2380-sync-skills, getGlobalConfigDir (Copilot)) fail at base too,
and base additionally fails folded:bug-3288-model-catalog-install-path, which
this tree does not.

tests/copilot-install.test.cjs

@@ -1362,7 +1362,10 @@ const EXPECTED_AGENTS = listAgentFiles().length;
 const { PROBE_TIMEOUT_MS, INSTALL_TIMEOUT_MS } = require('./helpers/timeouts.cjs');
 
 function runCopilotInstall(cwd) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode([INSTALL_PATH, '--copilot', '--local', '--no-sdk'], {
     cwd,
@@ -1374,7 +1377,10 @@ function runCopilotInstall(cwd) {
 }
 
 function runCopilotUninstall(cwd) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode([INSTALL_PATH, '--copilot', '--local', '--uninstall', '--no-sdk'], {
     cwd,
@@ -1673,7 +1679,10 @@ describe('E2E: Copilot uninstall verification', () => {
 // ─── E2E: Copilot global scope (#786) ──────────────────────────────────────────
 
 function runCopilotInstallGlobal(cwd, configDir) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode(
     [INSTALL_PATH, '--copilot', '--global', '--config-dir', configDir, '--no-sdk'],
@@ -1684,7 +1693,10 @@ function runCopilotInstallGlobal(cwd, configDir) {
 }
 
 function runCopilotUninstallGlobal(cwd, configDir) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode(
     [INSTALL_PATH, '--copilot', '--global', '--config-dir', configDir, '--uninstall', '--no-sdk'],
@@ -1734,7 +1746,10 @@ describe('E2E: Copilot global install (#786)', () => {
 // ─── Claude uninstall: user file preservation (#1423) ─────────────────────────
 
 function runClaudeInstall(cwd) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode([INSTALL_PATH, '--claude', '--local', '--no-sdk'], {
     cwd,
@@ -1746,7 +1761,10 @@ function runClaudeInstall(cwd) {
 }
 
 function runClaudeUninstall(cwd) {
-  const env = { ...process.env };
+  // #3156: sandbox HOME — the installer writes <home>/.gsd/defaults.json via
+  // os.homedir() directly, which no env scrub can reach. See installSpawnEnv.
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const env = installSpawnEnv();
   delete env.GSD_TEST_MODE;
   const r = runNode([INSTALL_PATH, '--claude', '--local', '--uninstall', '--no-sdk'], {
     cwd,