refactor(db): remove dead DIRECT_TRANSACTION_METADATA_KEY code

The only consumer was in state.ts, which was removed in the prior commit.
Clean up the orphaned constant, import, and metadata usage in mutations.ts.

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

Author: KyleAMathews

packages/db/src/collection/mutations.ts

@@ -17,7 +17,6 @@ import {
   UndefinedKeyError,
   UpdateKeyNotFoundError,
 } from '../errors'
-import { DIRECT_TRANSACTION_METADATA_KEY } from './transaction-metadata.js'
 import type { Collection, CollectionImpl } from './index.js'
 import type { StandardSchemaV1 } from '@standard-schema/spec'
 import type {
@@ -231,9 +230,7 @@ export class CollectionMutationsManager<
     } else {
       // Create a new transaction with a mutation function that calls the onInsert handler
       const directOpTransaction = createTransaction<TOutput>({
-        metadata: {
-          [DIRECT_TRANSACTION_METADATA_KEY]: true,
-        },
+        metadata: {},
         mutationFn: async (params) => {
           // Call the onInsert handler with the transaction and collection
           return await this.config.onInsert!({
@@ -430,9 +427,7 @@ export class CollectionMutationsManager<
 
     // Create a new transaction with a mutation function that calls the onUpdate handler
     const directOpTransaction = createTransaction<TOutput>({
-      metadata: {
-        [DIRECT_TRANSACTION_METADATA_KEY]: true,
-      },
+      metadata: {},
       mutationFn: async (params) => {
         // Call the onUpdate handler with the transaction and collection
         return this.config.onUpdate!({
@@ -536,9 +531,7 @@ export class CollectionMutationsManager<
     // Create a new transaction with a mutation function that calls the onDelete handler
     const directOpTransaction = createTransaction<TOutput>({
       autoCommit: true,
-      metadata: {
-        [DIRECT_TRANSACTION_METADATA_KEY]: true,
-      },
+      metadata: {},
       mutationFn: async (params) => {
         // Call the onDelete handler with the transaction and collection
         return this.config.onDelete!({