docs: add AGENTS.md

Author: yoshinorin

AGENTS.md

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+* Respond to users in the language they use in their messages
+* Comments in source code are generally not required. When explanation is necessary, write comments in English.
+* Do not respond based on speculation or assumptions. If you must speculate or assume, declare it explicitly.
+* Keep explanations as concise as possible.
+* When writing implementations or tests, ensure there are no duplicates in the content.
+* When installing libraries or dependencies, explain which libraries or dependencies will be installed before implementation.
+* When changes or implementations are expected to be substantial, explain what changes will be made beforehand.
+* When requirements are ambiguous, ask clarifying questions before implementation
+* When conducting research, provide primary sources when available.
+* Never reference, modify, or write to directories above the current directory where the agent is running.