feat(deps): detect/prevent child→parent dependency anti-pattern (bd-nim5)

This prevents a common mistake where users add dependencies from child
issues to their parent epics. This creates a deadlock:
- Child can't start (blocked by open parent)
- Parent can't close (children not done)

Changes:
- dep.go: Reject child→parent deps at creation time with clear error
- server_labels_deps_comments.go: Same check for daemon RPC
- doctor/validation.go: New check detects existing bad deps
- doctor/fix/validation.go: Auto-fix removes bad deps
- doctor.go: Wire up check and fix handler

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/doctor.go

@@ -400,6 +400,8 @@ func applyFixList(path string, fixes []doctorCheck) {
 			err = fix.MergeArtifacts(path)
 		case "Orphaned Dependencies":
 			err = fix.OrphanedDependencies(path)
+		case "Child-Parent Dependencies":
+			err = fix.ChildParentDependencies(path)
 		case "Duplicate Issues":
 			// No auto-fix: duplicates require user review
 			fmt.Printf("  ⚠ Run 'bd duplicates' to review and merge duplicates\n")
@@ -785,6 +787,11 @@ func runDiagnostics(path string) doctorResult {
 	result.Checks = append(result.Checks, orphanedDepsCheck)
 	// Don't fail overall check for orphaned deps, just warn
 
+	// Check 22a: Child→parent dependencies (anti-pattern, bd-nim5)
+	childParentDepsCheck := convertDoctorCheck(doctor.CheckChildParentDependencies(path))
+	result.Checks = append(result.Checks, childParentDepsCheck)
+	// Don't fail overall check for child→parent deps, just warn
+
 	// Check 23: Duplicate issues (from bd validate)
 	duplicatesCheck := convertDoctorCheck(doctor.CheckDuplicateIssues(path))
 	result.Checks = append(result.Checks, duplicatesCheck)