fix(local): use native shell on Windows

[ruirui6946]

What type of PR is this?

fix

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• The description of this PR title is user-oriented and clear enough for others to understand.
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(Optional) Translate the PR title into Chinese.

fix(local): 在 Windows 上使用原生 shell

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

Local.Execute and Local.ExecuteStreaming always started /bin/sh -c, so both paths failed on Windows before a command could run. This change centralizes shell selection: Windows uses cmd.exe /C, while Unix and macOS keep /bin/sh -c.

The regression test verifies the platform-specific arguments and executes the selected shell. On Windows, targeted synchronous and streaming cases pass for successful output, stderr, non-zero exit codes, missing commands, and stream completion.

Validation:

• go test -run '^(TestNewShellCmd|TestExecuteStreaming/(ExecuteStreaming_with_echo|ExecuteStreaming_with_command_failure|ExecuteStreaming_with_stderr_output|ExecuteStreaming_with_empty_command|ExecuteStreaming_with_normal_completion)|TestExecute/(empty_command|non-zero_exit_code|non-zero_exit_code_with_stderr|command_not_found))$' ./...
• The full Windows suite still stops in pre-existing Unix-specific tests: TestLsInfo/path_is_a_file,_not_a_directory and TestGrepRaw/grep_successfully (hard-coded /tmp/test).

zh(optional):

Local.Execute 和 Local.ExecuteStreaming 原先都固定调用 /bin/sh -c，导致 Windows 在命令启动前直接失败。本修复统一选择系统 shell：Windows 使用 cmd.exe /C，Unix 和 macOS 保持 /bin/sh -c。

(Optional) Which issue(s) this PR fixes:

Fixes #896

(optional) The PR that updates user documentation:

N/A