chore: add instructions for coding agents and Copilot review

[michalsn]

Description
This PR adds tool-neutral repository guidance in AGENTS.md and focused GitHub Copilot instructions primarily for code review, aligned with CodeIgniter compatibility, testing, documentation, database, and Worker Mode requirements.

This is experimental and can be reverted if it does not improve AI-assisted contributions and Copilot review quality, or if it causes regressions.

Ref: https://docs.github.com/en/copilot/concepts/agents/code-review#choosing-between-custom-instructions-agentsmd-and-skills

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[memleakd]

Would it make sense to use a root-level AGENTS.md for the repository-wide guidance instead of making it specific to Copilot? Copilot supports AGENTS.md, as do many other agent harnesses.

With more contributors using different AI assisted workflows, I believe a tool neutral instruction file could help those tools follow CI4's standards and conventions more consistently and reduce avoidable review work.

[michalsn]

Good point. I've moved the repository-wide guidance to a root-level AGENTS.md and kept the Copilot-specific files focused on review behavior and path-specific instructions. This should make the shared guidance useful across different AI-assisted workflows.