feat: pack skills from remote Git repositories

[pavelanni]

Summary

Currently skillctl pack only works with local directories. Add
support for packing skills directly from a Git repository URL:

skillctl pack https://github.com/acme/agent-skills/tree/main/code-review

This enables the distribution vehicle workflow: pull skills from
various upstream sources (GitHub, GitLab, etc.), curate them, and
package them as signed OCI images.

Context

• MLflow Skill Registry RFC uses source_type: "git" with repo URLs
  as skill sources. Being able to pack directly from those URLs closes
  the loop.
• Supports the multi-source bundle workflow: pull skills from
  different repos and combine them into a single OCI bundle image.

Considerations

• Git authentication (SSH keys, tokens)
• Pinning to a specific commit/tag for reproducibility
• Sparse checkout (only the skill subdirectory, not the full repo)