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

[jelharou]

What does this PR do?

Follow-up to #325. PR #320 changed SiteNode.children from embedded BuildingNode | ItemNode objects to flat string[] IDs. #325 updated the runtime call sites (rehydrate-site-children.ts, generate-variants.ts, nodes/site/renderer.tsx) but missed the three scene templates in packages/mcp/src/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 when running bun test --cwd packages/mcp (part of .github/workflows/mcp-ci.yml):

✗ scene templates > empty-studio template nodes all pass AnyNode.safeParse
✗ scene templates > two-bedroom template nodes all pass AnyNode.safeParse
✗ scene templates > garden-house template nodes all pass AnyNode.safeParse

error: ... failed AnyNode.safeParse at children.0: Invalid input: expected string, received object

Each template already initialises site.children with the correct string id (e.g. ['building_empty']); the obsolete mutation blocks just need to be removed.

How to test

1. bun test --cwd packages/mcp — confirm all 281 tests pass (previously 3 failing).
2. Open the editor, load any of the three templates (empty-studio, two-bedroom, garden-house) — confirm the scene loads and the site/building/level hierarchy renders correctly.

Screenshots / screen recording

N/A — non-visual fix (schema alignment in template data).

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch