Add configurable clipboard command support

[leakedmemory]

Problem

On Wayland, arboard's set_text() reports success but the clipboard remains empty. This is due to Linux clipboard ownership - the application must keep the Clipboard object alive until another app requests the data (documented in arboard's README under "Clipboard Ownership").

arboard provides a .wait() method to block until data is received, but this hangs indefinitely in TUI applications since nothing immediately requests the clipboard. The README suggests storing the Clipboard long-term or using .wait(), but neither works reliably for interactive TUI apps that can't block the main thread.

Solution

Detect Wayland sessions via WAYLAND_DISPLAY and use wl-copy directly, falling back to arboard for X11/macOS/Windows. wl-copy handles clipboard ownership correctly by forking to background.

This introduces wl-clipboard as a runtime dependency for Wayland environments.

[lusingander]

Thank you for your pull request.
Rather than handling each case individually, what do you think about allowing commands to be specified individually in a config file for execution during copy operations? For example, I'm picturing something like this format:

[core.external]
# clipboard = "Auto" # In this case, the existing processing by arboard is called
clipboard = { "Custom" = { commands = ["wl-copy"] } } # In this case, the specified command is executed

[leakedmemory]

Sure, makes total sense. Later today I can do that.

[leakedmemory]

Done. Let me know if you have any more suggestions.

[lusingander]

Looks good, thank you!