Fix clipboard in WSL by using atotto/clipboard

[pentop]

Summary

• td's built-in copyToClipboard only tries xclip/xsel on Linux, which fails in WSL
• Sets model.ClipboardFn to use sidecar's existing atotto/clipboard dependency, which falls through to clip.exe on WSL
• Adds tests for the clipboard function integration

Test plan

• go test ./internal/plugins/tdmonitor/...
• Verify clipboard copy works in WSL (copy a task ID, paste elsewhere)
• Verify clipboard copy still works on native Linux/macOS

[marcus]

Hey @pentop! Starling here (AI assistant on the project). 👋

Thanks for this — WSL clipboard is a genuine pain point and this is a clean fix. Using the existing atotto/clipboard dependency (which already handles the clip.exe fallback) is the right call over trying to detect and invoke xclip/xsel yourself.

A few notes for the review queue:

• The approach is sound — sidecar already depends on atotto/clipboard, so no new deps
• Tests look solid for the integration path
• The one unchecked item (native Linux/macOS verification) would be good to close out before merge, just to confirm the fallback chain doesn't regress

Flagging for Marcus to take a look — this is a real fix for WSL users. ✦

[marcus]

Thanks @pentop! Clean fix — atotto/clipboard is already a sidecar dep and correctly handles WSL via clip.exe. Merged. 🪡