Add AGENTS.md with coding agent instructions

[Copilot]

Adds machine-readable instructions for AI coding agents following the agents.md format specification.

Contents

• Build/Test Commands: cargo test, cargo fmt, cargo clippy with usage examples
• Project Structure: Entry points, core modules, test fixtures location
• Implementation Patterns: GitHub API pagination handling, event processing pipeline, multi-threaded repository queries
• Coding Conventions: Enum deserialization with unknown variant handling, lazy_static! for regex, debug formatting patterns
• Testing Practices: Inline unit tests, JSON fixtures via include_str!(), edge case coverage

Design Notes

References README.md for installation rather than duplicating content. Focuses on operational details relevant to code generation/modification that aren't appropriate for user-facing documentation.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Generate coding agents instructions to guide LLM for code generation 🤖</issue_title>
<issue_description>It should be concise and AGENTS.md format (https://agents.md) so that it is simple and model-agnostic.
Try to prevent duplication with actuel README.md so that future changes are not required in both files.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Generate coding agents instructions to guide LLM for code generation 🤖 #195

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.