fix/windows powershell utf8

[ysmla]

Issue for this PR

Closes #

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

修复opencode模型使用shell时的中文乱码问题
Closes #30205、#30055
If you paste a large clearly AI generated description here your PR may be IGNORED or CLOSED!

How did you verify your code works?

Opencode web中，模型使用shell输出中文的时候产生乱码如图：

发现PowerShell 5.1 管道输出用系统 OEM 代码页（CP936）编码、opencode 按 UTF-8 解码导致乱码；修复是在 Windows 上给 PowerShell 命令注入 [Console]::OutputEncoding UTF-8 前导语句，覆盖 shell 工具和 ! 命令两条路径，权限检查和界面显示不受影响，修复后如图：

Screenshots / recordings

No
If this is a UI change, please include a screenshot or recording.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• No issue referenced. Please add Closes #<number> linking to the relevant issue.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

Hey! Your PR title Fix/windows powershell utf8 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]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate PRs Found

This PR appears to be directly related to previous Windows PowerShell UTF-8 encoding fixes:

1. fix(core): force UTF-8 for Windows shells #36608 - fix(core): force UTF-8 for Windows shells
   fix(core): force UTF-8 for Windows shells #36608

2. fix: set default UTF-8 encoding for spawned subprocess on Windows #31658 - fix: set default UTF-8 encoding for spawned subprocess on Windows
   fix: set default UTF-8 encoding for spawned subprocess on Windows #31658

Why they're related: All three PRs address the same core issue—fixing UTF-8 encoding problems for PowerShell output on Windows. PR #49012 mentions it's related to issues #30205 and #30055, and appears to implement a similar fix by injecting [Console]::OutputEncoding = UTF-8 into PowerShell commands.

You should verify whether these previous PRs already resolved the issue or if this is addressing a gap they missed.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new pull request that follows our guidelines.