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/cmd/add.go

@@ -43,7 +43,7 @@ var addCmd = &cobra.Command{
 			input.Rels = append(input.Rels, service.RelInput{Type: models.RelType("_prio::" + cPrio), Target: ""})
 		}
 		if cNamespace != "" {
-			input.Rels = append(input.Rels, service.RelInput{Type: models.RelInNamespace, Target: cNamespace})
+			input.Namespace = cNamespace
 		}
 		if cAssignee != "" {
 			input.Rels = append(input.Rels, service.RelInput{Type: models.RelAssignee, Target: cAssignee})