chore(cleanup): purge /command residuals — fakeEngine, canon golden, openapi

Follow-up tidy after the cross-service /command removal (#73):

- Rename the router test double dummyExecutor -> fakeEngine (and the
  newExecutor / setupRouterExecutor helpers -> newFakeEngine /
  setupRouterEngine): it implements handler.Engine now, "executor" was a
  leftover of the removed adapter. Test-only.

- Regenerate the ui/core canon signing golden onto user.games.order
  (request_user_games_command.json -> request_user_games_order.json, fresh
  canonical bytes + Ed25519 signature) and drop the last
  user.games.command references from the Go/TS tests and docs.

- Align game openapi: CommandRequest.cmd no longer carries minItems: 1. It
  is now used only by PUT /api/v1/order, which accepts an empty batch
  (clearing the player's stored order, equivalent to removing every
  command); the contract test freezes the empty-allowed shape.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

game/openapi_contract_test.go

@@ -275,7 +275,8 @@ func TestGameOpenAPISpecFreezesCommandRequest(t *testing.T) {
 
 	cmdSchema := schema.Value.Properties["cmd"]
 	require.NotNil(t, cmdSchema, "CommandRequest.cmd schema must exist")
-	require.Equal(t, uint64(1), cmdSchema.Value.MinItems, "CommandRequest.cmd minItems must be 1")
+	require.Zero(t, cmdSchema.Value.MinItems,
+		"CommandRequest.cmd must allow an empty batch — an empty order clears the player's stored order")
 }
 
 func TestGameOpenAPISpecFreezesGetBattleOperation(t *testing.T) {