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/service/api_client.go

@@ -72,6 +72,9 @@ func (c *apiClient) GetByID(id string) (*models.Node, error) {
 
 func (c *apiClient) List(filter ListFilter) ([]*models.Node, error) {
 	q := url.Values{}
+	if filter.Namespace != "" {
+		q.Set("namespace", filter.Namespace)
+	}
 	for _, r := range filter.Rels {
 		if r.Target == "" {
 			q.Add("rel", string(r.Type))