fix(sync): run gstack-brain-sync through bash, not cmd.exe, on Windows

[ShahriarLak]

The bug

On Windows the brain-sync stage of /sync-gbrain fails on every run:

'C:\Users\...\.claude\skills\gstack\bin\gstack-brain-sync' is not recognized as an internal or external command,
operable program or batch file.

  ERR   brain-sync   gstack-brain-sync exited 1 (0.0s)

Why the #1731 fix didn't cover it

#1731 added shell: NEEDS_SHELL_ON_WINDOWS to these spawns. That is correct for the gbrain.cmd shim and does nothing here: shell: true routes through cmd.exe, which resolves .cmd/.bat via PATHEXT but has no concept of a shebang, so an extension-less bash script is rejected outright.

A .cmd shim needs a shell. A shebang script needs an interpreter. The two cases look identical and are not, which is why one fix appeared to cover both.

Why it matters more than one red line

artifacts_sync_mode defaults to pushing curated artifacts to git. With this stage dead, a Windows user's learnings accumulate in ~/.gstack and are never committed, while the sync report shows one red line among otherwise green stages. It reads as a minor stage failure and is actually silent data stagnation.

The change

New bashScriptInvocation() in lib/gbrain-exec.ts:

• Resolves Git for Windows' bash.exe and passes the script as argv[0].
• Prefers Git bash over a bare bash on PATH: WindowsApps ships a bash.exe that is the WSL launcher, and if it wins PATH order it reads C:\... as a Linux path and the script never sees the repo.
• Forward-slashes the script path, since bash treats backslashes as escapes.
• GSTACK_BASH overrides for unusual installs.
• Returns null when no bash exists, so the stage reports skipped (no bash found; set GSTACK_BASH to your Git bash.exe) rather than an unactionable spawn error.

POSIX is unchanged: the script is exec'd directly, no interpreter, no shell.

Tests

The #1731 tripwire asserted the shape that does not work (spawnSync(brainSyncPath, ...) + shell: NEEDS_SHELL_ON_WINDOWS), so it now asserts the opposite and fails if a raw spawn returns. Six unit tests cover the resolver: POSIX passthrough, Git-bash resolution, path slashing, shell: false, GSTACK_BASH override, and the no-bash null.

The gbrain.cmd assertions in that file are untouched — that part of #1731 is right and still enforced.

Verification (Windows 11, gstack 1.61.0.0, bun 1.3.11)

Before: ERR brain-sync gstack-brain-sync exited 1 (0.0s)
After: OK brain-sync curated artifacts pushed (4.2s) — and the artifacts repo committed and pushed on its own.

The suite is nondeterministic on Windows (the same tree gave 743 and 741 failures on back-to-back runs), so I compared the affected files rather than the whole run: 14 failures before, 14 after, with 6 new passing tests. No behaviour change on POSIX, where the invocation is byte-identical to today's.

🤖 Generated with Claude Code

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