docs: add Windows bash troubleshooting

[dashitongzhi]

Summary

• Expand Windows bash troubleshooting docs with the actual detection order Codebuff uses
• Document supported Windows shell setups: PowerShell/CMD with Git for Windows, Git Bash, and WSL
• Add diagnostics for PATH/Git Bash/WSL confusion and document CODEBUFF_GIT_BASH_PATH workarounds

References #819.

Test plan

• Docs-only change; no code executed.
• Reviewed WINDOWS.md for Markdown formatting and verified the diff is limited to documentation.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.