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_fix.go

@@ -227,6 +227,8 @@ func applyFixList(path string, fixes []doctorCheck) {
 			err = doctor.FixGitignore()
 		case "Redirect Tracking":
 			err = doctor.FixRedirectTracking()
+		case "Last-Touched Tracking":
+			err = doctor.FixLastTouchedTracking()
 		case "Git Hooks":
 			err = fix.GitHooks(path)
 		case "Daemon Health":