fix(required-gate): isolate skipped label concurrency

[ArchonVII]

Summary

• Put skipped non-ci:full label-only runs in a separate label-skip-* concurrency group.
• Keep normal PR events and ci:full label changes in the real required-gate group with cancellation enabled.
• Document and test that skipped label runs cannot replace pending real gate runs.

Verification

• npm test

Vitest reported 6 files passed / 122 tests passed.

• Workflow syntax

actionlint examples/repo-required-gate.yml .github/workflows/repo-required-gate.yml returned no diagnostics.
git diff --check returned no whitespace errors; only existing CRLF conversion warnings were printed.

Docs / Changelog

Added .changelog/unreleased/61-label-skip-concurrency.md and updated README required-gate label semantics.

Linked Issue

Closes #61

Risks

Low. This only changes the example caller concurrency group expression; reusable workflow internals are unchanged.