fix(mcp): templates use string IDs for SiteNode.children

PR #320 changed SiteNode.children from embedded BuildingNode/ItemNode
objects to flat string[] IDs. PR #325 updated the runtime call sites
but missed the three scene templates, which still mutated the site
node's children array to embed full building objects after building
the flat dict. This caused AnyNode.safeParse to fail for site_empty,
site_2br, and site_garden in bun test --cwd packages/mcp.

Remove the obsolete mutation blocks; each template already initialises
site.children with the correct string id (e.g. ['building_empty']).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/mcp/src/templates/empty-studio.ts

@@ -238,16 +238,6 @@ function buildTemplate(): SceneGraph {
     nodes[node.id as AnyNodeId] = node
   }
 
-  // SiteNode.children is a discriminatedUnion of BuildingNode/ItemNode objects
-  // (not string ids) — so the site must embed the full building node. The
-  // rest of the tree uses string ids per the BaseNode/LevelNode/WallNode
-  // schemas. We mutate the flat-dict copy of the site here so the nested
-  // representation round-trips through AnyNode.safeParse.
-  const siteInDict = nodes['site_empty' as AnyNodeId] as unknown as {
-    children: unknown[]
-  }
-  siteInDict.children = [nodes['building_empty' as AnyNodeId]]
-
   return {
     nodes,
     rootNodeIds: ['site_empty'] as AnyNodeId[],

packages/mcp/src/templates/garden-house.ts

@@ -263,10 +263,6 @@ function buildTemplate(): SceneGraph {
     ],
   } as unknown as AnyNode
 
-  // SiteNode.children is a discriminatedUnion of BuildingNode/ItemNode objects
-  // (not string ids) per the schema — embed the full building node here.
-  ;(nodes.site_garden as unknown as { children: unknown[] }).children = [nodes.building_garden!]
-
   return {
     nodes: nodes as Record<AnyNodeId, AnyNode>,
     rootNodeIds: ['site_garden'] as AnyNodeId[],

packages/mcp/src/templates/two-bedroom.ts

@@ -291,10 +291,6 @@ function buildTemplate(): SceneGraph {
     ],
   } as unknown as AnyNode
 
-  // SiteNode.children is a discriminatedUnion of BuildingNode/ItemNode objects
-  // (not string ids) per the schema — embed the full building node here.
-  ;(nodes.site_2br as unknown as { children: unknown[] }).children = [nodes.building_2br!]
-
   return {
     nodes: nodes as Record<AnyNodeId, AnyNode>,
     rootNodeIds: ['site_2br'] as AnyNodeId[],