feat(security): receipt core shell sinks

Wire the three core bash egress sinks through gstack-egress-lib.sh:

- gstack-telemetry-sync: the batch POST now writes the payload to a
  temp file, receipts those exact bytes fail-closed, and hands curl the
  SAME file. On refusal nothing is sent and the cursor does not
  advance, so the batch stays buffered for the next run. The HTTP
  status is recorded as the receipt outcome.
- gstack-update-check: fail-open receipts (warn + proceed) on the
  Supabase ping POST, both VERSION curls (via a local
  _receipted_version_fetch helper that skips non-network schemes), and
  git ls-remote. The ping receipt is written inside the backgrounded
  subshell, so it can never block the script's exit.
- gstack-brain-sync: fail-closed git-class receipts. The push receipt
  is written BEFORE the commit consumes the queue, so a refused receipt
  leaves the queue intact and the next run retries the whole drain
  (pinned by a new queue-intact-on-refusal test, including the
  problem/cause/fix refusal message shape). The retry-path fetch and
  retry push carry their own fail-closed receipts.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
(cherry picked from commit 3c60f699acceaf1c92a218874711e05fc17dca5d)

test/brain-sync.test.ts

@@ -330,6 +330,69 @@ describe('gstack-brain-sync secret scan', () => {
   });
 });
 
+// ---------------------------------------------------------------
+// Egress receipt gate: receipt-before-commit, queue intact on refusal
+// ---------------------------------------------------------------
+describe('gstack-brain-sync egress receipt gate', () => {
+  test('refused receipt leaves the queue intact, makes no commit, and next run retries', () => {
+    if (process.platform === 'win32' || process.getuid?.() === 0) return; // chmod is advisory there
+    run(['gstack-artifacts-init', '--remote', bareRemote]);
+    run(['gstack-config', 'set', 'artifacts_sync_mode', 'full']);
+    fs.mkdirSync(path.join(tmpHome, 'projects', 'p'), { recursive: true });
+    fs.writeFileSync(path.join(tmpHome, 'projects/p/learnings.jsonl'),
+      '{"skill":"x","insight":"y","ts":"2026-04-22T10:00:00Z"}\n');
+    run(['gstack-brain-enqueue', 'projects/p/learnings.jsonl']);
+    const commitsBefore = git(['rev-list', '--count', 'HEAD']).stdout.trim();
+
+    // Make the receipt unwritable: security dir exists but is read-only.
+    fs.mkdirSync(path.join(tmpHome, 'security'), { recursive: true, mode: 0o500 });
+    try {
+      const refused = run(['gstack-brain-sync', '--once']);
+      expect(refused.status).toBe(1);
+      // DX contract: problem + cause + fix, plain language.
+      expect(refused.stderr).toContain('NOT sent');
+      expect(refused.stderr).toContain('EGRESS_RECEIPT_FAILED');
+      expect(refused.stderr).toContain('Fix: chmod -R u+w');
+      expect(refused.stderr).toContain('ATTEMPTS to send off-machine');
+      // Queue intact (receipt is written BEFORE the commit consumes it).
+      const queue = fs.readFileSync(path.join(tmpHome, '.brain-queue.jsonl'), 'utf-8');
+      expect(queue).toContain('projects/p/learnings.jsonl');
+      // No local commit was created.
+      expect(git(['rev-list', '--count', 'HEAD']).stdout.trim()).toBe(commitsBefore);
+      // Nothing reached the remote.
+      const remoteLog = spawnSync('git', ['--git-dir=' + bareRemote, 'log', '--oneline'], { encoding: 'utf-8' });
+      expect(remoteLog.stdout).not.toMatch(/sync: 1 file/);
+      const status = JSON.parse(fs.readFileSync(path.join(tmpHome, '.brain-sync-status.json'), 'utf-8'));
+      expect(status.status).toBe('push_failed');
+      expect(status.message).toContain('EGRESS_RECEIPT_FAILED');
+    } finally {
+      fs.chmodSync(path.join(tmpHome, 'security'), 0o700);
+    }
+
+    // Next run (ledger writable again) drains the intact queue and pushes.
+    const retry = run(['gstack-brain-sync', '--once']);
+    expect(retry.status).toBe(0);
+    const log = spawnSync('git', ['--git-dir=' + bareRemote, 'log', '--oneline'], { encoding: 'utf-8' });
+    expect(log.stdout).toMatch(/sync: 1 file/);
+  });
+
+  test('successful push writes a git-class receipt before the send', () => {
+    run(['gstack-artifacts-init', '--remote', bareRemote]);
+    run(['gstack-config', 'set', 'artifacts_sync_mode', 'full']);
+    fs.mkdirSync(path.join(tmpHome, 'projects', 'p'), { recursive: true });
+    fs.writeFileSync(path.join(tmpHome, 'projects/p/learnings.jsonl'),
+      '{"skill":"x","insight":"y","ts":"2026-04-22T10:00:00Z"}\n');
+    run(['gstack-brain-enqueue', 'projects/p/learnings.jsonl']);
+    const r = run(['gstack-brain-sync', '--once']);
+    expect(r.status).toBe(0);
+    const ledger = fs.readFileSync(path.join(tmpHome, 'security', 'egress.jsonl'), 'utf-8');
+    const records = ledger.trim().split('\n').map((l) => JSON.parse(l));
+    const pushReceipt = records.find((rec) => rec.sink === 'brain-sync' && rec.payload_class === 'curated-memory-git-push');
+    expect(pushReceipt).toBeTruthy();
+    expect(pushReceipt.sha256).toBeNull(); // git owns the bytes
+  });
+});
+
 // ---------------------------------------------------------------
 // Uninstall preserves user data
 // ---------------------------------------------------------------