[Documentation] Add AGENTS.md for AI coding agents

[djalmaaraujo]

Related issue

Rui-9

Description

Adds an AGENTS.md file with project guidance for AI coding agents (Claude Code, Copilot, Cursor, etc.). This helps agents quickly understand:

• Project overview and tech stack (Phlex, Tailwind, Stimulus)
• Common development commands (tests, linting)
• Component architecture and patterns (Base class, file structure, test conventions)
• Generator system and dependency mapping
• CI setup and PR conventions

A CLAUDE.md file is also added, referencing AGENTS.md so Claude Code picks up the same guidance.

Testing instructions

• Verify AGENTS.md content is accurate against the codebase
• Verify CLAUDE.md contains only the @AGENTS.md reference