[fix]: make GitHub skills work cleanly on Windows, macOS, and Linux

Problem

GitHub-related skills in extensions still include POSIX/bash-specific setup patterns (/tmp, heredocs, mkdir -p, ar -czf, python3, sed, grep). Even after the filename fixes in #317, native Windows is still not an out-of-the-box experience.

Scope

• Update github, github-pr-review, github-pr-reviewer, and github-repo-monitor`n- Remove bash-only setup paths as the default
• Prefer shell-neutral instructions, Python/file-editor flows, or dual PowerShell + bash examples
• Avoid /tmp and other POSIX-only path assumptions

Goal

A user on Windows, macOS, or Linux can use these GitHub skills in Agent Canvas without WSL, Git Bash, or manual shell translation.

Out of scope

Automation runtime/bootstrap changes.
Likely this issue will take care of that https://github.com/OpenHands/automation/issues/178

This issue was created by an AI agent (OpenHands) on behalf of the user.
And the human user edited this too. And read it.

Locate the definitions for the github, github-pr-review, github-pr-reviewer, and github-repo-monitor skills, then inspect their setup instructions for /tmp, heredocs, mkdir -p, tar, python3, sed, and grep. Replace default POSIX-only paths with shell-neutral or cross-platform guidance, and verify that the four skills can be followed on Windows, macOS, and Linux without WSL or manual translation.