refactor(clipboard): share provider detection across y/p/paste/hist

Extracts _fish_clipboard_copy and _fish_clipboard_paste so the
wl-copy/xclip/win32yank fallback chain lives in one place instead of
four near-duplicates. hist now goes through the same chain, so it
also gets the xclip (X11) fallback it was missing before, alongside
win32yank on WSL2.

functions/_fish_clipboard_copy.fish

@@ -0,0 +1,28 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   _fish_clipboard_copy
+#
+# DESCRIPTION
+#   Copies stdin to the system clipboard. Tries wl-copy (Wayland), then
+#   xclip (X11), then win32yank.exe (WSL2).
+#
+# EXIT STATUS
+#   0  Text copied to clipboard
+#   1  No clipboard provider found
+#
+# EXAMPLE
+#   echo "hello" | _fish_clipboard_copy
+function _fish_clipboard_copy --description 'Copy stdin to the system clipboard'
+    if type -q wl-copy
+        wl-copy
+    else if type -q xclip
+        xclip -selection clipboard
+    else if type -q win32yank.exe
+        win32yank.exe -i --crlf
+    else
+        echo "Error: No clipboard provider (wl-copy, xclip, or win32yank) found." >&2
+        return 1
+    end
+end