feat(labctl): add caching for image downloads and hooks

[jmgilman]

Summary

• Add --cache-dir flag to labctl images sync for local download caching
• Create internal/cache package for managing cached files by checksum
• Pass LABCTL_HOOK_CACHE environment variable to hooks for artifact caching
• Update iso-to-container.sh to cache rootfs.tar by ISO hash
• Add actions/cache step to images-sync.yml workflow

Cache Structure

~/.cache/labctl/
├── downloads/     # Cached ISOs by checksum hash (12-char key)
└── hooks/
    └── vyos-integration-test/
        └── rootfs-{iso-hash}.tar

Expected CI Time Savings

Phase	Without Cache	With Cache
ISO Download (~400MB)	~60s	~0s
sqfs2tar (~2GB rootfs)	~90s	~5s
Total per image	~3 min	~45s

Test plan

• CI workflow runs successfully on this PR
• First run downloads and caches the ISO
• Second run uses cached ISO (look for "Using cached:" in logs)
• Hook caching works (look for "Using cached rootfs.tar:" in iso-to-container.sh output)

🤖 Generated with Claude Code