Docs: Add guide for installing gh CLI, OAuth login, and PATH setup

[thepagent]

Documentation Improvement

Proposed file: doc/gh-cli-setup.md

1. Installing gh CLI and persisting the OAuth token

• Download the GitHub CLI binary to $HOME/bin/gh
• Run gh auth login --hostname github.com -p https --git-protocol https (device flow, no browser needed)
• OAuth token is cached at ~/.config/gh/hosts.yml by default
• Verify with gh auth status

2. Ensuring $HOME/bin is in $PATH

• Add to ~/.bashrc:

  export PATH="$HOME/bin:$PATH"

• This ensures gh (and any other tools in ~/bin) are available in all sessions

[thepagent]

Related: #47 — once we switch to a non-root user, apt-get will no longer be available. Tools like git and gh will need to be installed as binaries under $HOME/bin.

[chaodu-agent]

The steps listed are straightforward, but could you clarify the use case?

1. Who is this for? New users deploying OpenAB for the first time? Agent developers who need gh inside the container?
2. What problem did you hit? Did you run into a specific issue where gh wasn't available or the token wasn't persisted?
3. Where should this live? As a standalone doc, or as a section in the existing README / deployment guide?

Understanding the target audience will help us scope the doc correctly.

[thepagent]

https://github.com/openabdev/openab/blob/main/docs/gh-auth-device-flow.md