docs: write AGENTS.md

Author: isuke

AGENTS.md

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+# AGENTS.md
+
+This document defines **project-specific assumptions, restrictions, and priorities** for AI agents working in this repository.
+It is intended to be read by both human contributors and AI assistants.
+
+---
+
+## 1. Communication
+
+- **Language**
+  - Conversations, explanations, and review comments may be in Japanese or English depending on the user’s preference.
+  - Code comments and documentation should follow the existing style in the repo and are **primarily written in English** (e.g. `README.md`).
+- **Tone**
+  - Keep explanations **concise and information-dense**.
+  - Always leave at least a short note about the motivation or background of non-trivial changes.
+
+---
+
+## 2. Project Overview (for AI)
+
+- This package is a **CLI tool to keep git commit messages consistent**.
+- Entry point: `lib/index.js` (wired via `bin.git-consistent`).
+- Main tech stack:
+  - Node.js ESM (`"type": "module"`)
+  - CLI: `commander`, `inquirer`, `prompt-sync`
+  - Configuration: `.git_consistent` (YAML) and `.gitcommit_template`
+  - Tests: `ava`
+  - Lint / formatting: `eslint` + `prettier`
+
+**Key files / directories**
+
+- `lib/` – CLI implementation
+- `.git_consistent` – sample configuration for user projects
+- `.gitcommit_template` – commit template
+- `test/` – AVA tests
+- `.github/workflows/main.yml` – CI (lint + test)
+
+---
+
+## 3. Change Policy and Priorities
+
+- **Do not break existing CLI behavior**
+  - When changing flags, prompts, or interactive flows, **update tests and documentation** accordingly.
+- **Treat user-facing surfaces as public API**
+  - Command-line options, configuration file format, and template variable names are de facto public API.
+  - If compatibility is intentionally broken, update `CHANGELOG.md` and the relevant sections of `README.md`.
+- **Prefer small and safe steps**
+  - Before large refactors, introduce internal helpers and additional tests to create a safety net.
+
+---
+
+## 4. Coding Guidelines (for AI-generated code)
+
+- **Style**
+  - Use **ES Modules** (`import` / `export`) to match existing code.
+  - Follow eslint/prettier for details like semicolons and indentation; do not introduce a custom manual style.
+- **Lint / Format**
+  - Write code with `npm run lint` in mind; avoid patterns that will obviously violate rules.
+  - Assume `npm run fix` exists for auto-fixes, but do not rely on it to hide poor structure.
+- **Tests**
+  - When adding or changing logic, usually add or update **AVA tests in `test/`**.
+  - Read existing test names and assertions carefully to avoid breaking their intent.
+
+---
+
+## 5. `.git_consistent` and `.gitcommit_template`
+
+- These files directly affect users’ commit workflows, so **prioritize compatibility and clarity**.
+- When changing samples:
+  - Ensure the examples are consistent with explanations in `README.md`.
+  - Ensure the configuration actually works with `git consistent` and produces sensible commit messages.
+
+---
+
+## 6. Documentation Rules
+
+- When changing CLI options or behavior:
+  - Update the relevant sections in `README.md` (for example the **Command Reference**).
+  - Add a short note to `CHANGELOG.md` describing the change (in English).
+- If only internal implementation details change and behavior stays the same:
+  - README updates are usually unnecessary, but you may add comments or tests to clarify intent.
+
+---
+
+## 7. Special Instructions for AI Agents
+
+- Avoid asking the user for clarification more than necessary; instead, **infer reasonable assumptions** from existing code and `README.md`.
+- For changes that might be controversial or breaking, leave clear reasoning:
+  - Potentially breaking changes in behavior or semantics
+  - Modifications to CLI interface or configuration format
+- When reasonable, add 1–2 lines explaining **why** a change was made, either here in `AGENTS.md`, in commit messages, or in code comments.