fix(exports): add ./package.json subpath and document ESM-only (#75)

Stays ESM-only (Node ≥20 already the floor, dual CJS would add build
complexity for no alpha benefit). Adds the missing ./package.json
subpath export (fixes typedoc and some bundlers) and a "default"
fallback condition on the main entry. Documents the ESM requirement
clearly in the README.

Closes #49

Co-authored-by: Claude <noreply@anthropic.com>

Author: zeevdr

README.md

@@ -12,6 +12,10 @@ TypeScript SDK for [OpenDecree](https://github.com/opendecree/decree) -- schema-
 
 > **Alpha** -- This SDK is under active development. APIs and behavior may change without notice between versions.
 
+## Requirements
+
+- Node.js **≥ 20** (ESM-only package — CommonJS is not supported)
+
 ## Install
 
 ```bash

package.json

@@ -8,8 +8,10 @@
   "exports": {
     ".": {
       "types": "./dist/index.d.ts",
-      "import": "./dist/index.js"
-    }
+      "import": "./dist/index.js",
+      "default": "./dist/index.js"
+    },
+    "./package.json": "./package.json"
   },
   "files": [
     "dist",