Clarify nested task grouping in the agent checklist format

AGENTS.md

@@ -45,6 +45,7 @@ All planned implementation work must be represented in `TODO.md`.
 
 - Use headings to group related work into sections.
 - Use checkbox items for each concrete task.
+- Group larger tasks in sub-sections of smaller tasks to keep the checklist organized.
 - Keep tasks small enough to complete and review independently.
 - Add new tasks when discoveries expand the scope.
 - When starting a task, claim it for the agent workflow by changing `[ ]` to `[-]`. Keep it `[-]`
@@ -61,6 +62,20 @@ Recommended format:
 ```md
 # TODO
 
+## Setup
+
+### Initial Setup
+
+- [x] Install required dependencies.
+- [-] Configure environment variables.
+- [ ] Verify initial project setup by running the basic test suite.
+
+### Environment Setup
+
+- [x] Set up local development environment.
+- [-] Configure project-specific environment variables.
+- [ ] Verify environment setup by running environment checks or sample scripts.
+
 ## Core Gameplay
 
 - [ ] Add player movement controls.