docs(skill): document glab orbit local in orbit skill

Summary

Update the orbit skill to document the glab orbit local command added in gitlab-org/cli!3226 (merged).

Changes to skills/orbit/SKILL.md:

• Updated frontmatter description to mention glab orbit local
• Bumped skill version 0.5.0 → 0.6.0
• Added new "Local CLI (glab orbit local)" section documenting:
  • Install/update/run lifecycle
  • Pass-through args
  • All config keys and the GLAB_ORBIT_LOCAL_BINARY_PATH env var
  • Platform support (macOS/Linux only)
  • When to use local vs remote

Validation

• markdownlint-cli2 skills/orbit/SKILL.md → 0 errors
• lychee --offline skills/orbit/SKILL.md → 3/3 links OK
• Ran 5 opencode-refine smoke prompts against the updated skill; all produced the expected answers (install with --install --yes, update with --update, pass-through glab orbit local index, custom binary via GLAB_ORBIT_LOCAL_BINARY_PATH / orbit_local_binary_path, scripted use via --yes or the orbit_local_auto_* config keys).

Related: gitlab-org/cli!3226 (merged) Closes https://gitlab.com/dgruzd/tasks/-/work_items/2653