feat: add agent instructions document with operational guidelines

AGENTS.md

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+# Agent Instructions
+
+This document outlines the operational guidelines and standards for agents working in this workspace.
+
+## 1. Language Standards
+
+- Code & Comments: All comments, documentation strings (docstrings), and variable names in source code files (everything outside the `docs/` directory) must be written in English.
+- Documentation: Files within the `docs/` directory may be written in the user's preferred language (e.g., Japanese).
+
+## 2. Package Management
+
+- Tool: Use uv as the Python package manager for this project.
+
+## 3. Task Documentation Workflow
+
+When receiving a request that is neither a simple question nor a single-step task, follow this workflow:
+
+### Create Task Document
+
+- Create a new markdown file in the `docs/tasks/` directory.
+- Naming convention: `YYYYMMDD_{sequence}.md` (e.g., `20251123_01.md`, `20251123_02.md`).
+- This file is referred to as the "Task Document".
+
+### Content Requirements
+
+- Language: Write the content in the language the user is currently using.
+- Breakdown: Decompose the requested task into specific, actionable steps.
+
+### User Confirmation
+
+- CRITICAL: After creating the Task Document and BEFORE starting any implementation work, you must present the plan to the user and ask for confirmation that the approach is correct.
+
+### Maintenance
+
+- Continuously update and edit the Task Document to reflect progress, changes in requirements, or additional information discovered during execution.