feat: add gh CLI to Dockerfile

[avoidwork]

Summary

Add the gh CLI to the Dockerfile as an explicit package dependency.

Motivation

The project and its skills rely heavily on the gh CLI for GitHub operations — issue creation, PR management, label handling, raw API calls. Currently the Docker image may have it installed implicitly or not at all. Making it an explicit dependency in the Dockerfile ensures reproducibility and guarantees the binary is present in every build.

Proposed Solution

• Add gh to the Dockerfile package installation step alongside other dependencies
• Follow the same installation pattern used for other CLI tools in the image
• Verify the image builds and gh --version works inside the container

What This Is Not

• Not a documentation-only change
• Not a homegrown GitHub API client
• Not a Node.js wrapper around the GitHub REST API

Environment

• OS: Linux 7.0.2-7-pve (Proxmox)
• madz version: 1.18.2

OpenSpec Note

This project uses OpenSpec for feature development. If this request is approved, I will:

1. Run /opsx:propose to generate a full proposal with specs and tasks
2. Iterate on the design before any code is written
3. Follow the task-driven implementation workflow

Additional Context

The skills collectively use gh for: issue CRUD, PR creation/editing/merging, label management, and raw gh api calls. All of these execute gh as a shell command via the terminal tool. This change makes the dependency explicit in the build image rather than leaving it to chance.