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/models/node.go

@@ -89,7 +89,7 @@ func (n *Node) AddRelation(relType RelType, target string) {
 	if n.Relations == nil {
 		n.Relations = make(map[string][]string)
 	}
-	if relType == RelAssignee || relType == RelCreated || relType == RelInNamespace {
+	if relType == RelAssignee || relType == RelCreated {
 		n.Relations[string(relType)] = []string{target}
 		return
 	}

src/models/rel_type.go

@@ -8,13 +8,12 @@ type Rel struct {
 }
 
 const (
-	RelBlocks      RelType = "blocks"
-	RelSubtask     RelType = "subtask"
-	RelRelated     RelType = "related"
-	RelCreated     RelType = "created"
-	RelAssignee    RelType = "assignee"
-	RelInNamespace RelType = "in_namespace"
-	RelMentions    RelType = "mentions"
+	RelBlocks   RelType = "blocks"
+	RelSubtask  RelType = "subtask"
+	RelRelated  RelType = "related"
+	RelCreated  RelType = "created"
+	RelAssignee RelType = "assignee"
+	RelMentions RelType = "mentions"
 
 	// Permission rels (subject → object). Levels are inclusive and transitive.
 	RelCanRead      RelType = "can_read"       // level 1: visible in list/show