[skill] Make the formatter work on Windows

Makes skill/scripts/format.mjs work on Windows.

• Direct-run check: import.meta.url.endsWith(process.argv[1].split("/").at(-1)) never matches a Windows backslash path, so the CLI exits 0 without output. It now compares real paths, which still works when the skill folder is installed as a symlink or junction. If argv[1] isn't a file, it's treated as not a direct run instead of throwing.
• Media references: localRef returned ./captures\before.png on Windows. It now emits / separators there. Other platforms are unchanged.

Verification:

• Windows 11: pnpm test goes from 25/31 to 31/31.
• Linux (WSL, Node 24): pnpm test 31/31, and pnpm verify:skill passes.
• I haven't run verify:local or the GitHub smokes, and the commits aren't signed. Squash-merging would give you a signed commit.