feat: Add GitHub CLI setup with xterm.js-safe auth #55

Problem

Coding agents (Claude Code, OpenCode, etc.) frequently need gh CLI for GitHub operations — creating PRs, viewing issues, cloning repos. Currently users must install and authenticate manually inside the terminal, and the default gh auth login uses interactive arrow-key menus that break in browser-based xterm.js PTY environments.

Proposed Solution (cherry-pick from PR #52)

PR #52 includes GitHub CLI setup that is independent of the OpenCode fork:

1. Automated gh installation during the setup phase
2. xterm.js-safe auth wrapper that avoids interactive arrow-key selection menus (which don't work in browser PTY)
3. New setup step in app.py parallel agent setup

Why separate from PR #52

This is a standalone setup script + a new step in app.py. It doesn't require the OpenCode fork, spawner app, or performance fixes. Extracting it keeps the review focused.

Files affected

• setup_github.py (new) — install and configure gh CLI
• app.py — add setup step