chore(labels): restore canonical label source file

[ss-o]

Summary

• add lib/labels.yml as the canonical machine-readable label source referenced by the org runbooks
• mirror the current live z-shell/.github label set exactly
• include conservative legacy migration mappings and sync-policy metadata for future dry-run rollout tooling

Validation

• parsed with Ruby YAML.load_file
• confirmed 31 labels and 31 unique names
• confirmed all required runbook labels are present
• compared against live z-shell/.github labels; no missing or extra labels
• signed commit verified locally with SSH signing

Notes

• This does not change repository labels.
• This does not run any org-wide label migration.
• Future work should add dry-run sync tooling before applying labels across child repos.

Refs #411

[Copilot]

Pull request overview

Restores a canonical, machine-readable source of truth for the z-shell organization’s GitHub label taxonomy, including legacy label migration mappings and conservative sync policy metadata to support future automation.

Changes:

• Adds lib/labels.yml defining the canonical label set (name/color/description).
• Adds legacy_migrations mappings to translate historical labels into the canonical taxonomy.
• Adds sync_policy metadata to guide safe, non-destructive rollout behavior.