Add skopeo support to mise configuration

## Summary
Add skopeo container image tool to mise-en-place configuration for consistent tool version management.

## Background
During the migration from devbox to mise-en-place (#5), skopeo was identified as not being available in the mise registry. This issue tracks finding and implementing a solution to manage skopeo via mise.

## Current State
- skopeo is commented out in .mise.toml with note about registry unavailability
- Tool is currently managed via system package manager or manual installation

## Investigation Needed
- [ ] Research third-party mise plugins for skopeo
- [ ] Investigate using ubi backend for GitHub releases installation
- [ ] Check if skopeo can be managed via aqua backend
- [ ] Consider creating custom mise plugin if needed

## Potential Solutions

### Option 1: ubi Backend
Check if skopeo releases on GitHub can be managed via:
```toml
skopeo = "ubi:containers/skopeo"
```

### Option 2: Third-party Plugin
Search for community mise plugins that support skopeo

### Option 3: Custom Plugin
Create a custom mise plugin for skopeo if no existing solution works

### Option 4: System Package Manager Integration
Document system package manager installation in setup scripts as fallback

## Acceptance Criteria
- [ ] skopeo can be installed and managed via mise
- [ ] Version consistency across development environments
- [ ] Integration works with existing dotfiles setup
- [ ] Documentation updated with chosen approach

## Related
- Closes TODO.md item about skopeo alternatives
- Part of overall mise migration (#5)

Start with the commented skopeo entry in .mise.toml and the related TODO.md item, then investigate the ubi backend, aqua backend, and third-party plugin options described here. Done means skopeo is installable and version-managed through the mise configuration, works with the dotfiles setup, and the chosen approach is documented.

Written by the indexing model from the issue text.