New ui.copy plugin API for writing to the clipboard

What

• A new $.ui.copy plugin-facing function copies text to the clipboard using OSC 52 (a terminal escape sequence for clipboard access), restricted to the terminal surface. It declines with a reason (no-surface or no-clipboard) when there's no attached UI or the text is too long for an OSC 52 write.
• A matching ui.copy control-protocol action was added to the client, sending the text and an optional surface identifier over the same channel used for actions like ui.focus.
• ui.copy was added to the capability list used by the plugin/hooks worker sandbox, alongside existing capabilities like ui.open, ui.close, ui.panes, ui.blit, fs.read, and fs.write.

Why This gives plugins and hooks a sanctioned way to put text on the user's clipboard, gated by the same capability and sandbox system as other UI actions, instead of needing a workaround.