test(#3156): pin the ambient-HOME leak the scrub set cannot reach

Two halves, and the second is the one that matters.

The contract half asserts installSpawnEnv() and installerEnv() both replace the
ambient HOME, keep USERPROFILE tracking it (os.homedir() reads that one on
Windows), and still let an explicit override win.

The behavioural half drives the REAL installer against a REAL ambient HOME: it
points process.env.HOME at a canary, runs `install.js --cursor --local`, and
asserts the canary gains no .gsd. No assertion about the scrub set can stand in
for this, because writeNonClaudeDefaults() resolves through os.homedir(), which
reads no GSD variable -- a set-membership test would pass with the bug fully
live.

Negative-controlled against the pre-fix tree rather than assumed: reverting
installerEnv() to `{ ...process.env, ...overrides }` fails BOTH halves, the
behavioural one reporting the actual artifact
("the installer wrote GSD's user store into the ambient HOME: defaults.json").

tests/helpers-process-isolation.test.cjs

@@ -342,3 +342,69 @@ describe('#2665 round 4: the skillsHome walk is reversion-sensitive', () => {
     void out; // exit 0 is the assertion; execFileSync throws on nonzero
   });
 });
+
+describe('#3156: a raw installer spawn cannot write into the ambient HOME', () => {
+  const fs = require('node:fs');
+  const os = require('node:os');
+  const { execFileSync } = require('node:child_process');
+  const { installSpawnEnv } = require('./helpers.cjs');
+  const { installerEnv } = require('./helpers/install-shared.cjs');
+  const INSTALL_PATH = path.join(__dirname, '..', 'bin', 'install.js');
+
+  // Contract half — cheap, and it names the precedence the callers depend on.
+  test('the sandbox HOME replaces the ambient one, but an explicit override still wins', () => {
+    for (const build of [installSpawnEnv, installerEnv]) {
+      const env = build();
+      assert.notStrictEqual(env.HOME, process.env.HOME,
+        'a raw installer spawn must not inherit the ambient HOME');
+      assert.strictEqual(env.USERPROFILE, env.HOME,
+        'USERPROFILE must track HOME — os.homedir() reads it on Windows');
+      assert.strictEqual(build({ HOME: '/explicit', USERPROFILE: '/explicit' }).HOME, '/explicit',
+        'an explicit HOME override must still win (overrides spread last)');
+    }
+  });
+
+  // Behavioural half — the one that actually fails pre-fix.
+  //
+  // bin/install.js writeNonClaudeDefaults() (#2834) writes
+  // <os.homedir()>/.gsd/defaults.json for every NON-Claude runtime, reading no
+  // GSD variable at all. So this is deliberately driven through the real
+  // installer against a real ambient HOME: no assertion about the scrub set can
+  // stand in for it, because no scrub set can reach os.homedir().
+  //
+  // Negative control: revert installerEnv() to `{ ...process.env, ...overrides }`
+  // and the canary gains .gsd/defaults.json.
+  test('installing a non-Claude runtime leaves the ambient HOME untouched', () => {
+    const canaryHome = fs.mkdtempSync(path.join(os.tmpdir(), 'gsd-3156-canary-home-'));
+    const projectDir = fs.mkdtempSync(path.join(os.tmpdir(), 'gsd-3156-project-'));
+    const realHome = process.env.HOME;
+    const realUserProfile = process.env.USERPROFILE;
+    try {
+      // Make the AMBIENT home the canary — the vector is the parent process's
+      // own HOME, exactly as on a developer machine or a CI runner.
+      process.env.HOME = canaryHome;
+      process.env.USERPROFILE = canaryHome;
+
+      execFileSync(process.execPath, [INSTALL_PATH, '--cursor', '--local', '--no-sdk'], {
+        cwd: projectDir,
+        encoding: 'utf-8',
+        stdio: ['pipe', 'pipe', 'pipe'],
+        env: installerEnv(),
+        timeout: 120_000,
+      });
+
+      assert.ok(!fs.existsSync(path.join(canaryHome, '.gsd')),
+        `the installer wrote GSD's user store into the ambient HOME: ${
+          fs.existsSync(path.join(canaryHome, '.gsd'))
+            ? fs.readdirSync(path.join(canaryHome, '.gsd')).join(', ')
+            : ''
+        }`);
+    } finally {
+      if (realHome === undefined) delete process.env.HOME; else process.env.HOME = realHome;
+      if (realUserProfile === undefined) delete process.env.USERPROFILE;
+      else process.env.USERPROFILE = realUserProfile;
+      fs.rmSync(canaryHome, { recursive: true, force: true });
+      fs.rmSync(projectDir, { recursive: true, force: true });
+    }
+  });
+});