ML-173: replace aws-sdk with aws4 + built-in fetch

Replace @aws-sdk/client-s3 (~150 transitive deps, ~14MB) with aws4
(0 transitive deps, ~40KB) and Node.js built-in fetch+crypto for
S3 ListObjectsV2 against Hetzner Object Storage.

- New s3-client.ts: SigV4 signing via aws4, regex-based XML parsing
  with namespace tolerance, pagination support, 5 error categories
- 29 unit tests (node:test, zero-dependency) covering signing,
  parsing, entities, missing fields, pagination, abort, errors
- Settlement guard for double done(null) on abort
- XML entity decoding and regex metacharacter escaping
- Dependency surface: 150 packages → 1 (aws4, single-file, auditable)

.pi/extensions/s3-browser/tsconfig.json

@@ -0,0 +1,15 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "ES2022",
+    "moduleResolution": "bundler",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "noEmit": true,
+    "allowImportingTsExtensions": true,
+    "lib": ["ES2022", "DOM"]
+  },
+  "include": ["s3-client.ts", "aws4.d.ts"],
+  "exclude": ["node_modules", "dist"]
+}