test: /review hardening — NOT-READY env isolation, workdir cleanup, perf

Applied from the adversarial subagent pass during /review on this branch:

- test/benchmark-cli.test.ts — new "NOT READY path fires when auth env
  vars are stripped" test. The default dry-run test always showed OK on
  dev machines with auth, hiding regressions in the remediation-hint
  branch. Stripped env (no auth vars, HOME→empty tmpdir) now force-
  exercises gpt + gemini NOT READY paths and asserts every NOT READY
  line includes a concrete remediation hint (install/login/export).
  (claude adapter's os.homedir() call is Bun-cached; the 2-of-3 adapter
  coverage is sufficient to exercise the branch.)

- test/taste-engine.test.ts — session-cap test rewritten to seed the
  profile with 50 entries + one real CLI call, instead of 55 sequential
  subprocess spawns. Same coverage (FIFO eviction at the boundary), ~5s
  faster CI time. Also pins first-casing-wins on the Geist/GEIST merge
  assertion — bumpPref() keeps the first-arrival casing, so the test
  documents that policy.

- test/skill-e2e-benchmark-providers.test.ts — workdir creation moved
  from module-load into beforeAll, cleanup added in afterAll. Previous
  shape leaked a /tmp/bench-e2e-* dir every CI run.

- test/publish-dry-run.test.ts — removed unused empty test/helpers
  mkdirSync from the sandbox setup. The bin doesn't import from there,
  so the empty dir was a footgun for future maintainers.

- test/helpers/providers/gpt.ts — expanded the inline comment on
  `--skip-git-repo-check` to explicitly note that `-s read-only` is now
  load-bearing safety (the trust prompt was the secondary boundary;
  removing read-only while keeping skip-git-repo-check would be unsafe).

Net: 45 passing tests (was 44), session-cap test 5s faster, one real
regression surface covered that didn't exist before.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/publish-dry-run.test.ts

@@ -27,7 +27,6 @@ beforeEach(() => {
   // structure: copy the bin into sandbox/bin/, write a controlled skills.json at the root.
   sandbox = fs.mkdtempSync(path.join(os.tmpdir(), 'publish-sandbox-'));
   fs.mkdirSync(path.join(sandbox, 'bin'));
-  fs.mkdirSync(path.join(sandbox, 'test', 'helpers'), { recursive: true });
   binCopy = path.join(sandbox, 'bin', 'gstack-publish');
   fs.copyFileSync(BIN, binCopy);
   fs.chmodSync(binCopy, 0o755);