feat(mcp): implement 19 scene query and mutation tools

Scene querying: get_scene, get_node, describe_node, find_nodes, measure.
Scene mutation (undoable, atomic): apply_patch, create_level, create_wall,
place_item, cut_opening, set_zone, duplicate_level, delete_node.
Undo/redo: undo, redo.
Export: export_json, export_glb (not_implemented stub).
Validation: validate_scene, check_collisions.

Each tool has:
- Exported Zod input + output schemas
- register<Tool>(server, bridge) wiring function
- Bun test with happy-path + error-path coverage via InMemoryTransport

59 tool tests, all passing end-to-end through MCP protocol.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/mcp/src/tools/export-json.ts

@@ -0,0 +1,33 @@
+import type { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js'
+import { z } from 'zod'
+import type { SceneBridge } from '../bridge/scene-bridge'
+
+export const exportJsonInput = {
+  pretty: z.boolean().optional(),
+}
+
+export const exportJsonOutput = {
+  json: z.string(),
+}
+
+export function registerExportJson(server: McpServer, bridge: SceneBridge): void {
+  server.registerTool(
+    'export_json',
+    {
+      title: 'Export JSON',
+      description:
+        'Return the scene as a serialized JSON string. Pass `pretty: true` to indent with 2 spaces.',
+      inputSchema: exportJsonInput,
+      outputSchema: exportJsonOutput,
+    },
+    async ({ pretty }) => {
+      const scene = bridge.exportJSON()
+      const json = JSON.stringify(scene, null, pretty ? 2 : 0)
+      const payload = { json }
+      return {
+        content: [{ type: 'text' as const, text: JSON.stringify(payload) }],
+        structuredContent: payload,
+      }
+    },
+  )
+}