feat: add storage system with local disk and S3 providers

Introduces a provider-agnostic storage subsystem for file attachments.
Includes local disk and S3 backends, asset/attachment DB schemas, issue
attachment CRUD routes with multer upload, CLI configure/doctor/env
integration, and enriched issue ancestors with project/goal resolution.

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

server/src/storage/provider-registry.ts

@@ -0,0 +1,18 @@
+import type { Config } from "../config.js";
+import type { StorageProvider } from "./types.js";
+import { createLocalDiskStorageProvider } from "./local-disk-provider.js";
+import { createS3StorageProvider } from "./s3-provider.js";
+
+export function createStorageProviderFromConfig(config: Config): StorageProvider {
+  if (config.storageProvider === "local_disk") {
+    return createLocalDiskStorageProvider(config.storageLocalDiskBaseDir);
+  }
+
+  return createS3StorageProvider({
+    bucket: config.storageS3Bucket,
+    region: config.storageS3Region,
+    endpoint: config.storageS3Endpoint,
+    prefix: config.storageS3Prefix,
+    forcePathStyle: config.storageS3ForcePathStyle,
+  });
+}