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>

internal/rpc/server_labels_deps_comments.go

@@ -4,11 +4,25 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"strings"
 
 	"github.com/steveyegge/beads/internal/storage"
 	"github.com/steveyegge/beads/internal/types"
 )
 
+// isChildOf returns true if childID is a hierarchical child of parentID.
+// For example, "bd-abc.1" is a child of "bd-abc", and "bd-abc.1.2" is a child of "bd-abc.1".
+func isChildOf(childID, parentID string) bool {
+	_, actualParentID, depth := types.ParseHierarchicalID(childID)
+	if depth == 0 {
+		return false // Not a hierarchical ID
+	}
+	if actualParentID == parentID {
+		return true
+	}
+	return strings.HasPrefix(childID, parentID+".")
+}
+
 func (s *Server) handleDepAdd(req *Request) Response {
 	var depArgs DepAddArgs
 	if err := json.Unmarshal(req.Args, &depArgs); err != nil {
@@ -18,6 +32,15 @@ func (s *Server) handleDepAdd(req *Request) Response {
 		}
 	}
 
+	// Check for child→parent dependency anti-pattern (bd-nim5)
+	// This creates a deadlock: child can't start (parent open), parent can't close (children not done)
+	if isChildOf(depArgs.FromID, depArgs.ToID) {
+		return Response{
+			Success: false,
+			Error:   fmt.Sprintf("cannot add dependency: %s is already a child of %s (children inherit dependency via hierarchy)", depArgs.FromID, depArgs.ToID),
+		}
+	}
+
 	store := s.storage
 	if store == nil {
 		return Response{