Feature Request: Configurable Default Shell on Windows

Problem

Currently, Kimi Code CLI hardcodes Windows PowerShell as the default shell on Windows systems:

https://github.com/MoonshotAI/kimi-cli/blob/main/src/kimi_cli/utils/environment.py#L33-L35

if os_kind == "Windows":
    shell_name = "Windows PowerShell"
    shell_path = KaosPath("powershell.exe")

This means even when users launch kimi from Git Bash (or other Unix-like environments on Windows like MSYS2, Cygwin), the Shell tool still uses PowerShell for command execution.

Use Case

Many developers on Windows prefer using Git Bash for development because:

1. Tool chain compatibility: Unix tools like
   p, awk, sed, find, make, cmake work out of
   e box
2. Path handling: Unix-style paths (/c/projecrc) are more consistent across platforms
3. Script portability: Scripts written in Git
   sh can run on Linux/macOS/CI without modiftion
4. Development workflow: Most open-source projects assume a Unix-like environment

Proposed Solution

Add support for configuring the default shell
via:

Option 1: Environment Variable (Recommended)

export KIMI_DEFAULT_SHELL=bash  # or "powersh
ell", "cmd", "zsh", etc.
export KIMI_DEFAULT_SHELL_PATH=/usr/bin/bash
# optional: custom path

Option 2: Config File

# ~/.kimi/config.toml
<!--This is a translation content dividing line, the content below is generated by machine, please do not modify the content below-->
---
## Problem
Currently, Kimi Code CLI hardcodes Windows PowerShell as the default shell on Windows systems:

https://github.com/MoonshotAI/kimi-cli/blob/main/src/kimi_cli/utils/environment.py#L33-L35

```python
if os_kind == "Windows":
    shell_name = "Windows PowerShell"
    shell_path = KaosPath("powershell.exe")

This means even when users launch kimi from Git Bash (or other Unix-like environments on Windows like MSYS2, Cygwin), the Shell tool still uses PowerShell for command execution.

Use Case

Many developers on Windows prefer using Git Bash for development because:

1. Tool chain compatibility: Unix tools like
   p, awk, sed, find, make, cmake work out of
   e box
2. Path handling: Unix-style paths (/c/projecrc) are more consistent across platforms
3. Script portability: Scripts written in Git
   sh can run on Linux/macOS/CI without modiftion
4. Development workflow: Most open-source projects assume a Unix-like environment

Proposed Solution

Add support for configuring the default shell
via:

Option 1: Environment Variable (Recommended)

export KIMI_DEFAULT_SHELL=bash  # or "powersh
ell", "cmd", "zsh", etc.
export KIMI_DEFAULT_SHELL_PATH=/usr/bin/bash
# optional: custom path

Option 2: Config File

# ~/.kimi/config.toml

Start with src/kimi_cli/utils/environment.py around lines 33-35 to trace how the Windows shell is selected and how the selected path is used. Compare the proposed environment-variable and config-file options against the existing CLI configuration flow; done means the default shell can be selected on Windows, including a custom path, without changing the current default.