test: cover powershell clipboard escaping

[T1mn]

Summary

• add a dedicated PowerShell escaping helper for clipboard writes (backticks and quotes)
• reuse the helper in the Windows clipboard path to match recent PowerShell fixes
• add a unit test to guard the escaping behavior

Testing

• bun test packages/opencode/test/clipboard.test.ts

[github-actions]

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

Based on my search results, there are no duplicate PRs that are addressing the same issue. PR #7177 is the only PR specifically focused on PowerShell clipboard escaping tests.

However, there are some related clipboard PRs in the repository:

• PR fix(clipboard): add OSC 52 fallback for SSH/tmux environments #7028: "fix(clipboard): add OSC 52 fallback for SSH/tmux environments" - Related clipboard work but addressing a different feature
• PR fix: validate DIBv5 format before fallback to DIB in Windows clipboard #2861: "fix: validate DIBv5 format before fallback to DIB in Windows clipboard" - Another Windows clipboard fix, but different scope

No duplicate PRs found