feat: replace in_namespace relation with ownership-based namespace membership

Remove the in_namespace edge relation. A node now belongs to a namespace if that
namespace has has_ownership on it. This simplifies the model: namespace membership
is determined by the ownership chain rather than a separate relation type.

Changes:
- Remove RelInNamespace constant
- Add Namespace fields to AddInput, UpdateInput, and ListFilter
- Update Add() to resolve namespace from input and assign it as owner
- Update List() to filter by namespace ownership instead of in_namespace edges
- Update() can now transfer nodes between namespaces via ownership transfer
- Remove in_namespace self-references from ensureNamespace/ensureGlobalNamespace

The ownership chain now fully describes both permissions and namespace membership,
reducing redundancy. All tests pass with the new model.

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

src/serve/server.go

@@ -77,7 +77,7 @@ func (s *server) listNodes(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 	if v := q.Get("namespace"); v != "" {
-		filter.Rels = append(filter.Rels, service.RelInput{Type: models.RelInNamespace, Target: v})
+		filter.Namespace = v
 	}
 	if v := q.Get("assignee"); v != "" {
 		filter.Rels = append(filter.Rels, service.RelInput{Type: models.RelAssignee, Target: v})