[BUG] /copy command: clipboard copy not working on Windows 11

Description

The /copy command reports success ("Copied to clipboard (N characters, N lines)") but the content is not actually placed on the system clipboard on Windows 11.

Steps to reproduce

1. Run a prompt that produces a multi-block response (e.g., two email drafts in separate code blocks)
2. Run /copy
3. The command outputs: Copied to clipboard (2122 characters, 43 lines)
4. Attempt to paste — clipboard is empty or contains old content

Expected behavior

The last response is copied to the system clipboard so it can be pasted into another application.

Actual behavior

The command reports success but the clipboard does not contain the response content.

Environment

• OS: Windows 11 Home 10.0.26200
• Shell: PowerShell 7 (pwsh)
• Claude Code: CLI