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/node_service.go

@@ -34,20 +34,23 @@ type NodeService interface {
 // Type is "prefix::value"), and edge rels (Target is a node name or ID).
 // The service applies defaults (type=issue, status=open for issues) and validates.
 type AddInput struct {
-	Title   string
-	Content string
-	DueDate string
-	Rels    []RelInput
+	Title     string
+	Content   string
+	DueDate   string
+	Namespace string // namespace name or ID; defaults to the user's personal namespace
+	Rels      []RelInput
 }
 
 // UpdateInput describes changes to apply to an existing node.
 // AddRels and RemoveRels accept both tag rels (Target == "") and edge rels.
 // Setting _status::done in AddRels is rejected when the node has open blockers.
-// Adding assignee or in_namespace rels replaces the previous single target.
+// Adding an assignee rel replaces the previous single target.
+// Setting Namespace transfers ownership from the current namespace to the new one.
 type UpdateInput struct {
 	Title      *string
 	Content    *string
 	DueDate    *string // nil = no change; pointer to "" = clear due date
+	Namespace  *string // nil = no change; namespace name or ID to move node into
 	AddRels    []RelInput
 	RemoveRels []RelInput
 }
@@ -56,6 +59,7 @@ type UpdateInput struct {
 // Tag filters (Target == "") match by rel_name prefix.
 // Edge filters (Target != "") are resolved to node IDs.
 type ListFilter struct {
+	Namespace  string // when non-empty, only return nodes owned by this namespace
 	Rels       []RelInput
 	HasDueDate bool // when true, only return nodes that have a due date set
 	DueWithin  *int // when non-nil, only return nodes due within this many days (includes overdue)