[WIP] fix: respect SHELL environment variable on Windows for bash commands

[nielpattin]

On Windows, when SHELL env was set (e.g., to Git Bash), LLM-executed bash commands and user shell commands would ignore it and use cmd.exe instead.

Changes:

• Strip Windows executable extensions (.exe/.cmd/.bat) from shell name
  detection so bash.exe is correctly matched to bash invocation rules
• Explicitly pass SHELL env to spawn() on Windows instead of relying on
  shell: true which defaults to cmd.exe
• This restores support for Git Bash and other POSIX shells on Windows

Tested with nushell and bash.exe from Git For Windows

Required for user that using Git Bash:

• Set SHELL env to C:\Program Files\Git\usr\bin\bash.exe
• C:\Program Files\Git\usr\bin need to be in path so the bash can use others linux utilities command.

Here is the video of using with Git Bash

gitbash-test.mp4

Here is the video of using with Nushell

nushell-test.mp4