Refactor: add agent training and repository conventions

[SolfE]

Summary

Add AGENTS.md to define the repository's training workflow and contribution conventions.

Problem

There was no repository-level guide describing how this project should be used as a backend training ground and portfolio project, or how the agent should coach, review, and structure iterative learning cycles.

Solution

Introduce an AGENTS.md document that defines the agent role, coaching approach, training cycle, cycle completion criteria, transition criteria, output expectations, and Git/GitHub conventions.

Changes

• add repository purpose and agent role definition
• define coaching rules and top-down learning flow
• define learning cycle start trigger and completion criteria
• define fixed outputs for each learning cycle
• document branch, commit, PR, and issue conventions based on existing repo patterns

Example

A future session can now start with the phrase 새로운 학습 시작하자, and the agent will treat it as the trigger for a new guided learning cycle.

Related Issues

N/A