test: revert GSTACK_HOME injection in the free runner — shared mutable state

The first full run under the strict runner surfaced 12 failures with one
root cause: injecting a single throwaway GSTACK_HOME per invocation made
6,900 tests share a MUTABLE scratch home. gstack-config tests wrote keys
into it; relink and update-check tests then read them (e.g. relink saw
skill_prefix left behind by a config test and produced prefixed names).
All 12 pass when run directly.

TMPDIR isolation stays (mkdtemp inside it is still per-call unique).
Tests needing GSTACK_HOME isolation mkdtemp their own per test — the
repo convention — and hermetic-env covers E2E children. The env-dump pin
now asserts GSTACK_HOME passes through UNTOUCHED so the injection can't
come back.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/test-free-shards.ts

@@ -446,12 +446,14 @@ function shardEpilogue(outcome: FreeShardOutcome, totalShards: number): string {
  * so tests can pin the summary-missing => failure backstop; fake passing
  * commands must print a synthetic `Ran N tests across M files. [Xms]` line.
  *
- * Per-shard state isolation: each spawned child gets its own throwaway
- * GSTACK_HOME and TMPDIR (TEMP/TMP on Windows) so shards — and the bun
- * --parallel workers inside the full-suite invocation — can't contend on the
- * operator's real ~/.gstack or trip over each other's temp files. Tests that
- * mkdtemp their own state dirs are unaffected: this only moves the DEFAULT
- * location. The throwaway dirs are removed when the shard finishes.
+ * Per-shard temp isolation: each spawned child gets its own throwaway TMPDIR
+ * (TEMP/TMP on Windows) so shards can't trip over each other's temp files.
+ * Deliberately NOT GSTACK_HOME: injecting one shared scratch home for a whole
+ * invocation made 6,900 tests share a MUTABLE state dir — config tests wrote
+ * keys into it and relink/update-check tests then read them (measured: 12
+ * cross-contamination failures on the first full run). Tests that need
+ * GSTACK_HOME isolation mkdtemp their own per test — the repo convention —
+ * and the hermetic-env machinery covers E2E children.
  */
 export async function runFreeShard(
   files: string[],
@@ -482,11 +484,8 @@ export async function runFreeShard(
 
   const env = { ...(options.env ?? process.env) };
   const stateDir = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-free-shard-'));
-  const gstackHome = path.join(stateDir, 'gstack-home');
   const childTmp = path.join(stateDir, 'tmp');
-  fs.mkdirSync(gstackHome);
   fs.mkdirSync(childTmp);
-  env.GSTACK_HOME = gstackHome;
   env.TMPDIR = childTmp;
   env.TEMP = childTmp;
   env.TMP = childTmp;

test/test-free-shards.test.ts

@@ -253,15 +253,18 @@ describe('test-free-shards: strict shard execution', () => {
     expect(lines.some((l) => /^\[test:free\] shard 7\/20: 0 files, 0s, pass$/.test(l))).toBe(true);
   });
 
-  test('each spawned shard gets its own throwaway GSTACK_HOME and TMPDIR, removed after the run', async () => {
+  test('spawned shard gets throwaway TMPDIR but NEVER an injected GSTACK_HOME', async () => {
+    // GSTACK_HOME injection was tried and reverted: one shared scratch home
+    // per invocation made 6,900 tests share MUTABLE state — config tests
+    // wrote keys that relink/update-check tests then read (12 measured
+    // cross-contamination failures). This pin keeps the regression out.
     const captureDir = fs.mkdtempSync(path.join(os.tmpdir(), 'free-shard-env-'));
     const dump = path.join(captureDir, 'env.json');
     try {
       const script =
         `const fs = require("fs");`
         + `fs.writeFileSync(${JSON.stringify(dump)}, JSON.stringify({`
-        + `  home: process.env.GSTACK_HOME, tmp: process.env.TMPDIR,`
-        + `  homeExists: fs.existsSync(process.env.GSTACK_HOME || ""),`
+        + `  home: process.env.GSTACK_HOME ?? null, tmp: process.env.TMPDIR,`
         + `  tmpExists: fs.existsSync(process.env.TMPDIR || "") }));`
         + `console.log(${JSON.stringify(SUMMARY_1)});`;
       const outcome = await runFreeShard(['env-dump'], 1, 1, {
@@ -271,13 +274,13 @@ describe('test-free-shards: strict shard execution', () => {
       });
       expect(outcome.status).toBe('passed');
       const seen = JSON.parse(fs.readFileSync(dump, 'utf8'));
-      expect(seen.home).toContain('gstack-free-shard-');
-      expect(seen.homeExists).toBe(true);
+      // GSTACK_HOME passes through untouched (whatever the parent had, incl. unset).
+      expect(seen.home).toBe(process.env.GSTACK_HOME ?? null);
+      // TMPDIR is a per-shard throwaway, cleaned up once the shard finishes.
+      expect(seen.tmp).toContain('gstack-free-shard-');
       expect(seen.tmpExists).toBe(true);
-      expect(seen.home).not.toBe(process.env.GSTACK_HOME ?? '');
       expect(seen.tmp).not.toBe(process.env.TMPDIR ?? '');
-      // The throwaway state dir is cleaned up once the shard finishes.
-      expect(fs.existsSync(seen.home)).toBe(false);
+      expect(fs.existsSync(seen.tmp)).toBe(false);
     } finally {
       fs.rmSync(captureDir, { recursive: true, force: true });
     }