docs: Add Copilot instructions for repository context

[Copilot]

Configures .github/copilot-instructions.md to provide GitHub Copilot with repository-specific context per best practices.

Key Sections

• Repository structure - Workspace layout (dfir_rs, hydro_lang, etc.) and purpose of each package
• Rust configuration - Dual toolchain setup: stable 1.90.0 for development, nightly for formatting/docs/WASM
• Testing - precheck.bash usage, snapshot tests (insta/trybuild), and WASM test requirements
• Code style - Workspace lints and clippy configuration
• Commit conventions - Conventional Commits format for squash+merge workflow

Includes common commands reference and clarifies Edition 2024 requirements.

[!WARNING]

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• docs.github.com
  • Triggering command: curl -s REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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.

<Onboard this repo></issue_description>

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

• Fixes hydro-project/hydro#2239

---

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

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