app: add AGENTS.md

bin/app/AGENTS.md

@@ -0,0 +1,8 @@
+* Run `make fmt` to format code.
+* Run `make compile-dev` to test desktop target.
+* Run `make compile-apk` to test mobile target.
+* Never use `crate::` inline in code. Always do an appropriate import.
+  Imports must always be clustered together so no duplicate `use crate::` lines.
+* `use parking_lot::Mutex as SyncMutex;` is the canonical sync mutex.
+  `use async_lock::Mutex as AsyncMutex;` is the canonical async mutex.
+