Avoid systeminfo when launching Fork's rebase editor

[camjac251]

Summary

• Detect wslpath directly instead of starting cmd.exe /c systeminfo.
• Detect WSL through WSL_INTEROP, with the historical kernel-name check as
  a fallback for older environments.
• Validate the editor path and rebase-todo argument before conversion.
• Propagate chmod and wslpath failures instead of launching the editor
  with an invalid path.
• Add shell regressions for conversion, failure propagation, and custom WSL
  kernels whose uname output omits Microsoft.

Problem

Fork.RI only needs WSL interoperability and wslpath, but it previously
started a Windows systeminfo scan and parsed the Windows build number before
every interactive rebase.

That indirect probe adds an unnecessary Windows process to a latency-sensitive
editor path. A kernel-name-only replacement is also insufficient because
custom WSL kernels can omit Microsoft from uname while interoperability is
fully available.

Using WSL_INTEROP for environment detection and checking wslpath directly
tests the capabilities that the script actually needs.

Verification

• bash -n resources/Fork.RI tests/fork-ri.sh
• shellcheck resources/Fork.RI tests/fork-ri.sh
• bash tests/fork-ri.sh
• Host smoke test on a custom WSL kernel whose uname output omits
  Microsoft
• git diff --check develop...HEAD
• Start an interactive rebase from Fork and confirm the rebase-todo editor
  opens without invoking cmd.exe /c systeminfo

Compatibility

The existing FORK_RI_EXE_PATH normalization and Windows-path conversion
remain in place. The script still launches the configured Fork.RI.exe with a
single converted rebase-todo path.

WSL_INTEROP is preferred when present. The historical Microsoft kernel-name
match remains as a fallback. Missing wslpath retains
exit status 1, while missing editor configuration or an invalid argument count
returns exit status 2.

Related issues

Related to #153. This removes the cmd.exe /c systeminfo invocation reported
there, but it does not claim to resolve unrelated failures while launching the
Windows editor executable itself.