Use cmd-based VS Code launcher on win32 in /diff and /files.
Show explicit error details when open fails.
In /diff, surface git difftool failure and troubleshooting hint.
Refs #2186
fullRender wrote \x1b[3J\x1b[2J\x1b[H (clear scrollback, then clear
screen). But \x1b[2J pushes on-screen content into scrollback, undoing
the preceding \x1b[3J. Reorder to \x1b[2J\x1b[H\x1b[3J so the screen is
cleared first and scrollback is wiped last.