Add AGENTS.md and CLAUDE.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: silverwind

AGENTS.md

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+# Instructions of agents
+
+- Use `make` to run development tasks
+- Before committing files, ensure that related `lint` tasks in `make` pass
+- Never force-push to pull request branches
+- Always start issue and pull request comments with an authorship attribution

CLAUDE.md

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+@AGENTS.md