Add code design rules for AI

.agents/references/code-design.md

@@ -0,0 +1,11 @@
+---
+name: code-design
+description: Design rules for implementation code in the MongoDB Java Driver. Use when adding or modifying an internal program element, or changing the implementation of a public API program element.
+---
+# Code Design
+
+## Executors
+
+- When instantiating an executor, prefer the `MongoThreadPoolExecutor` and `MongoScheduledThreadPoolExecutor` implementations.
+- Any task executed, submitted, or scheduled via an executor must not allow an `Exception` to be propagated;
+  `Error`s should generally not be caught, but if they are, they must still be propagated.

AGENTS.md

@@ -25,6 +25,7 @@ The default branch is `main` (not `master`). Always use `main` when comparing, d
 - Preserve existing comments — only remove if provably incorrect
 - No rewrites without explicit permission
 - When stuck or uncertain: stop, explain, propose alternatives, ask
+- When authoring or reviewing changes: consult the relevant `.agents/references/` file for each area the changes touch
 
 ## Build
 
@@ -71,6 +72,12 @@ public API classes must be thread-safe unless annotated otherwise.
 See [`.agents/references/api-design`](.agents/references/api-design.md) for stability annotations,
 design principles, and the full nullability and thread safety conventions.
 
+## Code Design
+
+Applies to implementation code — internal packages, method bodies, and private or package-access program elements.
+
+See [`.agents/references/code-design`](.agents/references/code-design.md) for the rules.
+
 ## Do Not Modify Without Human Approval
 
 - Wire protocol / authentication handshakes (`com.mongodb.internal.connection`)