fix(test): invoke bin/gstack-paths via bash (Windows shebang fix)

Round 13 of windows-free-tests fixes. Round 12 (scope pivot) revealed all
8 gstack-paths tests fail on Windows because the test invokes the bash
shebang script directly:

  spawnSync(BIN, [])  # BIN = path.join(ROOT, 'bin', 'gstack-paths')

Windows CreateProcess can't parse `#!/usr/bin/env bash` from the file.
The script never runs on Windows via this invocation path.

Fix: change to `spawnSync('bash', [BIN], ...)`. This matches production
usage — the script is sourced from inside skill bash blocks via
`eval "$(~/.claude/skills/gstack/bin/gstack-paths)"`, where bash is
always the executor. Mac/Linux behavior is identical (bash invocation
of a bash script).

Verified locally: 8/8 tests still pass on macOS.

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

test/gstack-paths.test.ts

@@ -5,8 +5,13 @@ import * as path from 'path';
 const ROOT = path.resolve(import.meta.dir, '..');
 const BIN = path.join(ROOT, 'bin', 'gstack-paths');
 
+// Invoke via `bash` rather than executing the shebang-script directly.
+// On Windows, spawnSync(scriptPath, ...) goes through CreateProcess, which
+// doesn't parse `#!/usr/bin/env bash`. Production usage always sources the
+// helper from inside a bash block (`eval "$(~/.claude/skills/gstack/bin/gstack-paths)"`)
+// so bash is always the executor — this matches that contract.
 function run(env: Record<string, string | undefined>): Record<string, string> {
-  const result = spawnSync(BIN, [], {
+  const result = spawnSync('bash', [BIN], {
     env: { PATH: process.env.PATH, ...env } as Record<string, string>,
     encoding: 'utf-8',
   });
@@ -70,7 +75,7 @@ describe('gstack-paths', () => {
   });
 
   test('output is shell-evalable: only KEY=VALUE lines, no extra prose', () => {
-    const result = spawnSync(BIN, [], {
+    const result = spawnSync('bash', [BIN], {
       env: { PATH: process.env.PATH, HOME: '/tmp/h' } as Record<string, string>,
       encoding: 'utf-8',
     });