feat(doctor): add check for last-touched file tracking

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>

cmd/bd/doctor.go

@@ -465,6 +465,11 @@ func runDiagnostics(path string) doctorResult {
 	result.Checks = append(result.Checks, vestigialWorktreesCheck)
 	// Don't fail overall check for vestigial worktrees, just warn
 
+	// Check 14f: last-touched file tracking (runtime state shouldn't be committed)
+	lastTouchedTrackingCheck := convertWithCategory(doctor.CheckLastTouchedNotTracked(), doctor.CategoryGit)
+	result.Checks = append(result.Checks, lastTouchedTrackingCheck)
+	// Don't fail overall check for last-touched tracking, just warn
+
 	// Check 15: Git merge driver configuration
 	mergeDriverCheck := convertWithCategory(doctor.CheckMergeDriver(path), doctor.CategoryGit)
 	result.Checks = append(result.Checks, mergeDriverCheck)