chore: remove stale files (#23)

Remove files that are no longer used:

* AGENTS.md: outdated
* HISTORY.md: the changelog now lives in CHANGELOG.md, generated by
git-cliff.
* test-results/.last-run.json: build artifact that should not be
tracked.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Frando

.gitignore

@@ -11,3 +11,5 @@
 /.agents
 /plans
 /REVIEW.md
+/test-results
+/AGENTS.md