ci: retire the obsolete sync-labels caller #4

Your feedback for the existing codebase

Remove only .github/workflows/sync-labels.yml as the fzy canary for z-shell/.github#472. That coordinated removal requires this repository's independently verified deliverable.

At main 4e5d14b, the file contains only a Tuesday 02:22 UTC schedule, manual dispatch and a single reusable job calling z-shell/.github/.github/workflows/sync-labels.yml@main. That target is absent. The caller is currently disabled_inactivity; this is a static missing-target defect, not evidence of current weekly failures.

The organization workflow conventions retire this pattern. Deliberate label maintenance remains owned by the label runbook.

Acceptance criteria

• Remove only .github/workflows/sync-labels.yml from main through a reviewed pull request.
• Preserve every other tracked file, including .github/workflows/trunk-check.yml and package metadata; preserve existing labels and settings.
• Verify the exact deletion, run applicable repository checks and record their results and limitations.
• Verify the merged main tree lacks the caller before proceeding to another repository under ci: 33 repositories schedule a sync-labels workflow that no longer exists .github#472.

Rollback is a reviewed revert restoring blob b9b9843c5af99539537e3f23ec3ddaf974b4ac76. The cross-repository issue remains open for the other callers.

Completed by PR #5 at d15e979d1cc2be9df92035c061b416db7e814b92. The merged main tree was verified to lack the caller, and all 12 retained tracked file blobs and modes match the original head. All acceptance criteria are met. Further removals remain owned by z-shell/.github#472.