docs: add critical note about .github templates to AGENTS.md

[PlayerNguyen]

Summary

Add a critical note in AGENTS.md reminding that PR and issue creation must follow the templates in .github/.

Related Issue

None

Type

• Documentation

Scope

• Root / config

Changes

• Add bold warning at top of AGENTS.md about using .github/ templates

Motivation

Make sure contributors (including AI agents) are aware that they must use the provided templates when creating issues and pull requests.

Testing

N/A

Checklist

• No console.log left in production code
• No secrets or keys committed
• Path aliases used correctly (@/, @common/)
• New code has tests (if applicable)
• API changes update OpenAPI spec (if applicable)
• UI changes work with Mantine + Tailwind v4 (if applicable)