docs: add AGENTS.md for Claude Code context

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

AGENTS.md

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+# AGENTS.md
+
+> **Start every session by reading this file.**
+> This file outlines the operational protocols, coding standards, and architectural decisions for this FastAPI project.
+
+## 1. Agent Operational Protocols
+
+### 🧠 Work Patterns (Plan-Act-Reflect)
+* **Plan:** Before writing code, briefly outline your plan. Identify which files you will touch and what the side effects might be.
+* **Act:** Execute the changes in small, atomic steps.
+* **Reflect:** After coding, verify your work. Did you break existing tests? Did you add new tests?
+
+### 🛡️ Git Discipline
+* **NEVER commit to `main` or `master` directly.** Always create a feature branch: `feature/your-feature-name` or `fix/issue-description`.
+* **Commit Messages:** Use the [Conventional Commits](https://www.conventionalcommits.org/) format.
+    * `feat: add user login endpoint`
+    * `fix: resolve database connection timeout`
+    * `refactor: split monolith dependency file`
+* **Atomic Commits:** Keep commits small. One logical change = one commit.
+
+### 📝 Changelog Maintenance
+* **Update `CHANGELOG.md`** with every user-facing change.
+* Format: `## [Unreleased] - YYYY-MM-DD` followed by `### Added`, `### Changed`, or `### Fixed`.
+
+---
+
+## 2. FastAPI Architecture & Best Practices
+*Reference: [FastAPI Best Practices](https://github.com/zhanymkanov/fastapi-best-practices)*
+
+### 📂 Project Structure (Directory-based, NOT File-type based)
+Do **not** group files by type (e.g., one huge `routers` folder). Group by **domain/module** inside a `src/` directory.
+
+**Correct Structure:**
+```text
+src/
+├── auth/
+│   ├── router.py      # Endpoints
+│   ├── schemas.py     # Pydantic models
+│   ├── service.py     # Business logic (CRUD, etc.)
+│   ├── dependencies.py# Module-specific dependencies
+│   └── config.py      # Module-specific settings
+├── posts/
+│   ├── router.py
+│   └── ...
+└── main.py            # App entry point