feat: support MSYS forward slashes with experimental flag

[pschiel]

WIP

issues:

• git rev-parse --show-toplevel returns E:/x/y forward slash format
• path.resolve(), path.relative(), realpathSync.native(), realpath (bash) break when git bash (msys) paths are used
  • /d/x within C drive (/c/) results in things like C:\d\x
  • relative paths not working like expected using cross-drive
  • some path matching issues (relative, contains, ..)
  • /tmp inside git bash is something else outside of it
• tools break using backslash paths (some backslashes get "eaten")
• escaping hell for agents
• observed in all win32 native shells

patch tries to fix:

• convert to forward slash format at all places where things break (tool calls and bash mostly)
• in Filesystem as wrapper functions (path.resolve -> Filesystem.resolvePath)
• configurable with flag (otherwise backslash paths are guaranteed - but still some bugs with these)

todo:

• clean up Filesystem functions (relativePath, overlaps, possibly add dirpath)
• add tests for Filesystem
• maybe change flag with SHELL detection
• test all tools, incl. permission checks

[github-actions]

Hey! Your PR title (feat) support MSYS forward slashes with experimental flag doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[Hona]

This sounds perfect @pschiel

If you can get it ready and finish the cleanup you said I can test and approve.

Btw don't put it behind an experimental flag. We'll ship once it's working.