fix(gbrain): probe CLI without command builtin

[jbetala7]

Fixes #1386

Summary

• remove the shell-based command -v gbrain probe from gstack-memory-ingest so Windows/Bun does not try to spawn the POSIX shell builtin as an executable
• use a direct gbrain --help probe in /sync-gbrain while keeping dry-run code-sync previews independent of local gbrain availability
• add focused regressions that prevent these paths from reintroducing command -v gbrain

Testing

• bun test test/gstack-gbrain-sync.test.ts
• bun test test/gstack-memory-ingest.test.ts
• git diff --check