docs: add Windows command references for skills

[neubig]

Summary

• add inline Windows/PowerShell notes only beside short POSIX-shell examples
• add scoped references/windows.md files only for skills with repeated Unix-style command blocks
• keep each reference focused on that skill's commands: env vars, curl, temp paths, tarballs, grep/sed, Linux package installs, SSH permissions, or gh quoting as applicable
• regenerate skills/index.js from the edited SKILL.md files

Validation

• npm run build:skills
• $env:PYTHONUTF8='1'; uv run --with pyyaml python scripts/sync_extensions.py --check commands catalog coverage
• verified every SKILL.md references/windows.md pointer resolves to a local file
• git diff --cached --check

Note: full sync_extensions.py --check is not useful from this Windows checkout because the symlink check reports existing .claude-plugin / .codex-plugin entries as non-symlinks. The coverage warnings shown by the scoped check are pre-existing and non-blocking.

Issue

Fixes #331

[neubig]

@all-hands-bot please review this PR.