✨ Add Copilot instructions for repository-specific guidance

[Copilot]

Why is this change necessary?

Enables GitHub Copilot coding agent to make context-aware decisions when working on issues by providing repository-specific conventions, boundaries, and workflows.

Description

Created .github/copilot-instructions.md following GitHub's best practices for Copilot coding agents. The file provides:

• Project context: Tech stack (Rust nightly, Cargo workspace), architecture (cmd/ for binaries, crates/ for libraries), and dependencies (ethrex, tokio, axum)
• Coding standards: rustfmt/clippy requirements, error handling patterns (thiserror in libs, anyhow at boundaries), type conventions
• Development workflow: Build/test commands (cargo test --workspace, just lint), CI requirements, Conventional Commits format
• Security boundaries: Explicit prohibitions (no secrets, no unsafe without SAFETY comments, no unwrap/expect outside tests)
• Common patterns: Guides for adding binaries, libraries, and dependencies following workspace structure

Note: Clarified that actual E2E test directory is tests/ (not test_data/ as referenced in PROJECT_STRUCTURE.md).

Closes #issue_number

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe):

Checklist

• I have tested my changes
• I have updated documentation if needed
• I have reviewed my code for best practices

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

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

• Fixes ✨ Set up Copilot instructions #279

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