test: make cross-project attribution shell setup portable

[NgoQuocViet2001]

Refs #765.

Fixes the Windows CI failure from the new cross-project attribution test added in 2e7a543.

What changed:

• keep /bin/bash on POSIX, but let Windows use the default shell instead of hardcoding /bin/bash
• normalize temp Windows paths to forward slashes before embedding them in Bash-style command fixtures

Why:

• Windows CI was failing with spawnSync /bin/bash ENOENT
• after that was fixed locally, the git -C fixture still misparsed C:\... paths as escapes and attribution fell back to the wrong project

Validation:

• npx vitest run tests/integration/cross-project-attribution.test.ts → 5 passed
• npm run typecheck
• npm run build