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/errors.ts

@@ -0,0 +1,34 @@
+import { ErrorCode, McpError } from '@modelcontextprotocol/sdk/types.js'
+
+/**
+ * Throw a structured MCP error. The SDK translates `McpError` into a
+ * JSON-RPC error response automatically.
+ */
+export function throwMcpError(code: ErrorCode, message: string, data?: unknown): never {
+  throw new McpError(code, message, data)
+}
+
+/**
+ * Return a non-throwing tool error payload — used for structured failures that
+ * we want the client to see inline in `content` rather than as a protocol
+ * error. Sets `isError: true` so SDK clients treat it as a failure.
+ */
+export function toolError(
+  message: string,
+  data?: Record<string, unknown>,
+): {
+  content: { type: 'text'; text: string }[]
+  isError: true
+} {
+  return {
+    content: [
+      {
+        type: 'text',
+        text: JSON.stringify({ error: message, ...(data ?? {}) }),
+      },
+    ],
+    isError: true,
+  }
+}
+
+export { ErrorCode, McpError }