scripts: win-shell-args mis-quotes an argument that ends in a backslash #5111

Problem

scripts/lib/win-shell-args.mjs quotes an argument for cmd.exe by wrapping it in "…" and doubling inner quotes. An argument that ends in a backslash (a directory path such as C:\Users\me\dir\) becomes "C:\Users\me\dir\", and the Windows C runtime's argument parser reads the final \" as an escaped quote, not the closing one, so the child receives a different argument (and the rest of the command line is swallowed into it). Backslashes immediately before an embedded quote have the same problem.

Dev tooling only: the callers are verify-skills-cli.mjs (the pinned validator, via npx) and claude-cli.mjs (skill evals) on Windows. Nothing published uses it. Found by Copilot on the v1.0.0 merge PR #5090.

Expected

Quoting follows the CRT rules: backslashes are doubled when they precede a " (including the closing one), with tests for a trailing backslash and for backslashes before an embedded quote.