Add native Windows text clipboard support

[amoreX]

Problem

copyToClipboard() supports pbcopy, xclip, and xsel, but has no Windows implementation. Clipboard operations silently do nothing on native Windows.

Acceptance criteria

• Use clip.exe or Set-Clipboard on Windows.
• Preserve existing macOS and Linux behavior.
• Return success/failure instead of silently swallowing every failure.
• Show useful fallback guidance when no clipboard mechanism is available.
• Add tests for each platform command.