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/e2e/e2e_permissions_test.go

@@ -168,10 +168,6 @@ func TestNamespaceExplicitCreate(t *testing.T) {
 
 	nsNode := env.parseNode(env.mustAx("add", "myworkspace", "--type", "namespace", "--json"))
 
-	if !nsNode.HasRelation("in_namespace", nsNode.ID) {
-		t.Errorf("expected namespace to have in_namespace pointing to itself, got relations: %v", nsNode.Relations)
-	}
-
 	users := env.parseNodes(env.mustAx("list", "--type", "user", "--json"))
 	var userNode *NodeResponse
 	for i := range users {