Add Codex-focused setup docs for WorkTrunk LLM commit generation and forge integration

[adrianmross]

Problem

The plugin includes WorkTrunk references, but it does not yet give Codex users an opinionated path for commit generation and GitHub integration.

Proposal

• Add docs for using Codex with WorkTrunk commit generation during wt merge / wt step commit
• Add guidance for GitHub CLI (gh) integration used by wt list CI status caching
• Document forge settings for non-default hostnames or SSH alias setups
• Add troubleshooting notes for Codex users who already have gh configured

Why

This would round out the plugin from basic worktree guidance into a more complete Codex + WorkTrunk workflow.