fixup! docs: document running tests without sharding in AGENTS.md

Author: alan-agius4

AGENTS.md

@@ -67,6 +67,7 @@ This is the source code for the Angular CLI and related build tooling. This guid
 - Use the `gh` CLI (GitHub CLI) for creating and managing pull requests.
 - **Fixup Commits:**
   - When addressing review feedback, **ALWAYS** use fixup commits (`git commit --fixup <commit>`) instead of amending existing commits. This preserves commit history during review and allows reviewers to easily see incremental changes.
+  - Only use fixup commits for changes that directly belong to the target commit. Unrelated changes must be made in a separate commit with their own commit message, not as a fixup commit.
   - Fixup commits are automatically squashed when merging with `pnpm ng-dev pr merge` or rebasing with `pnpm ng-dev pr rebase <pr>`.
 - Use `pnpm ng-dev pr` commands:
   - `pnpm ng-dev pr rebase <pr>`: Rebase a PR branch on its target branch and squash fixup commits.