Update AGENTS.md with upstream tracking requirements

- Added clarification that the API must not deviate from upstream jdk.sandbox repo
- Documented that bugs in main logic must be fixed upstream, not in this repo
- Only backporting machinery bugs may be fixed locally

Author: simbo1905

AGENTS.md

@@ -117,11 +117,15 @@ mvn exec:java -pl json-compatibility-suite -Dexec.args="--json"
 - `JsonBoolean`: JSON booleans.
 - `JsonNull`: JSON null.
 
+IMPORTANT: This API **MUST NOT** deviate from the upstream jdk.sandbox repo which is will track.  
+
 #### Internal Implementation (`jdk.sandbox.internal.util.json`)
 - `JsonParser`: Recursive descent JSON parser.
 - `Json*Impl`: Immutable implementations of `Json*` types.
 - `Utils`: Internal utilities and factory methods.
 
+IMPORTANT: Bugs in the main logic this code cannot be fixed in this repo they **MUST** be fixed upstream. Only bugs in any backporting machinery such as the double-check-locking class that is a polyfill for a future JDK `@StableValue` feature may be fixed in this repo. 
+
 ### Design Patterns
 - Algebraic Data Types: Sealed interfaces enable exhaustive pattern matching.
 - Immutable Value Objects: All types remain immutable and thread-safe.