docs: update agent guidelines with .sandbox usage

.gitignore

···
 #  option (not recommended) you can uncomment the following to ignore the entire idea folder.
 #.idea/
 .env
+
+.sandbox

AGENTS.md

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 ## Rules & Workflows
 
-- *todo*
+In general, before working on features locally, specs should have already been drafted
+by the developer.
+
+### Specs
+
+When drafting specs, let's use the `.sandbox` directory and format filenames with the
+following format:
+
+```sh
+.sandbox/spec_<feature>-<description>_YYYY-MM-DD.md
+```
+
+Each section should have an overview and `- [ ]` items for tasks.