Add Copilot instructions and automated environment setup for repository

[Copilot]

Configures repository-wide custom instructions for GitHub Copilot coding agents per the documented best practices, including automated dependency installation.

Changes

• .github/copilot-instructions.md: Comprehensive guidance covering:

  • Non-standard build system (cargo xtask required, not direct cargo commands)
  • Sysroot architecture (<repo>/sysroot/ for dev, ~/.flux for installed)
  • Project structure (15 workspace crates + 4 library packages)
  • Common failure modes and workarounds
  • CI pipeline validation steps
  • Note about automatic dependency installation in Copilot agent environment

• .github/workflows/copilot-setup-steps.yml: Automated environment setup workflow:

  • Pre-installs fixpoint via .github/actions/install-fixpoint
  • Pre-installs Z3 4.12.1 via cda-tum/setup-z3@v1.6.6
  • Runs automatically before Copilot coding agent starts working
  • Eliminates need for agents to discover and install dependencies via trial and error

Key Context Provided

Critical for agents unfamiliar with Flux:

• Build: Must use cargo xtask install (not cargo build)
• Test: Must use cargo xtask test (not cargo test)
• Sysroot: Libraries require pre-compilation; use cargo xtask build-sysroot after lib changes
• Dependencies: Automatically installed in Copilot agent environment; Z3 version matters (4.12.1 for standard tests, 4.15.3 for vtock/lean-demo)

Reduces exploration overhead by documenting tested workflows, architectural constraints, and repo-specific conventions upfront, while ensuring the agent environment is pre-configured with all required dependencies.

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 #1455

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

[nilehmann]

@copilot, add a copilot-setup-steps.yml following instructions here https://docs.github.com/en/enterprise-cloud@latest/copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment#preinstalling-tools-or-dependencies-in-copilots-environment. The workflow should install fixpoint and z3. Then remove instructions about dependencies from copilot-instructions.md because they will be installed.

[Copilot]

@copilot, add a copilot-setup-steps.yml following instructions here https://docs.github.com/en/enterprise-cloud@latest/copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment...

Done! Added .github/workflows/copilot-setup-steps.yml that installs fixpoint and Z3 4.12.1, and updated copilot-instructions.md to remove manual dependency installation instructions. Changes committed in 487eb14.