docs: scope AGENTS instructions by directory (#2083)

crates/AGENTS.md

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+# Crates Instructions
+
+Applies to all paths under `crates/`.
+
+## Library Design
+
+- Treat crate code as reusable library code by default.
+- Prefer `thiserror` for library-facing error types.
+- Do not use `unwrap()`, `expect()`, or panic-driven control flow outside tests.
+
+## Testing
+
+- Keep unit tests close to the module they test.
+- Keep integration tests under each crate's `tests/` directory.
+- Add regression tests for bug fixes and behavior changes.
+
+## Async and Performance
+
+- Keep async paths non-blocking.
+- Move CPU-heavy operations out of async hot paths with `tokio::task::spawn_blocking` when appropriate.