T8966: add legacy-label escape to invalid-task-id rule (commit check exempt) (#483)

The per-repo T-ID rule flagged every commit's first line, but on repos that
squash-merge AND block force-push (vyos.vyos enforces non_fast_forward on ~ALL
branches, zero bypass) authors cannot rewrite history to make existing commits
conform. The label became unclearable. Keep the title check (always fixable),
and exempt the per-commit check when a maintainer applies the new legacy
label. New PRs are still nudged toward the convention.

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