fix: Fork on Windows store forward-slash path (fixes #606)

[svarlamov]

fix: Fork on Windows store forward-slash path (fixes #606)

Summary

Fixes #606 by updating the Fork Windows installer integration to store and compare a valid Windows path with forward slashes (e.g. C:/Users/.../git.exe) for CustomGitInstancePath.

Introduces a new shared to_windows_git_bash_style_path() helper in src/mdm/utils.rs — a companion to the to_git_bash_path() helper added in PR #603. While to_git_bash_path produces MSYS-style paths (/c/Users/...) for tools that run inside git bash (e.g. Claude Code hooks), to_windows_git_bash_style_path produces valid Windows paths with forward slashes (C:/Users/...) for native GUI apps that store paths in JSON settings files.

Changes

• src/mdm/utils.rs: Added to_windows_git_bash_style_path(path) -> String — calls clean_path (strips \\?\ prefix) then replaces backslashes with forward slashes. Added two regression tests.
• src/mdm/git_clients/fork_app.rs: Fork's Windows check_client and install_prefs now use to_windows_git_bash_style_path (via a thin fork_custom_git_instance_path wrapper) instead of raw to_string_lossy(). Added a Windows-only regression test asserting the output is C:/... (not MSYS /c/...).
• src/mdm/git_clients/sublime_merge.rs: Refactored to use the same shared to_windows_git_bash_style_path helper instead of inline .replace('\\', "/"), for consistency and to also gain the clean_path prefix-stripping.

Review & Testing Checklist for Human

• On a Windows machine with Fork installed, run git-ai install-hooks and confirm Fork accepts the configured custom git path and the setting persists after restarting Fork.
• Verify %LOCALAPPDATA%\Fork\settings.json has CustomGitInstancePath set to a valid C:/.../git.exe path (not backslashes, not MSYS /c/...).
• Verify that re-running git-ai install-hooks is a no-op (i.e., check_client correctly detects "up-to-date"). Note: if an existing installation previously stored a backslash path, the first run after this change will re-write it — this is expected.
• Verify Sublime Merge still works correctly on Windows — the refactor to use to_windows_git_bash_style_path also adds clean_path prefix-stripping, which is a minor behavior change if paths ever had the \\?\ extended prefix.

Notes

• Requested by @svarlamov
• Link to Devin run
• This fix cannot be meaningfully end-to-end tested outside of a real Windows environment with Fork/Sublime Merge installed.
• The Fork-specific regression test is gated behind #[cfg(all(test, windows))], so it only runs on Windows CI runners.

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✅ Devin Review: No Issues Found

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[devin-ai-integration]

Good catch — Fork is a native Windows app, so CustomGitInstancePath should remain a valid Windows path (just with forward slashes). Updated fork_custom_git_instance_path to use clean_path(...).to_string_lossy().replace('\\', "/") and adjusted the Windows-only regression test accordingly.