Add bd doctor check that warns if .beads/last-touched is tracked by git. This file is local runtime state that should never be committed, as it causes spurious diffs in other clones. - CheckLastTouchedNotTracked() detects if file is git-tracked - FixLastTouchedTracking() untracks with git rm --cached - Comprehensive tests for all scenarios Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>