copilot-setup-steps: add Nix dev shell setup, CI cache restore, and Copilot instructions

[Copilot]

Sets up the Copilot coding agent environment for this repository with a Nix dev shell, CI cache integration, and agent instructions.

Changes Made

• New workflow file — creates .github/workflows/copilot-setup-steps.yml with steps to install Nix (nixbuild/nix-quick-install-action@v34) and warm the dev shell via nix develop -c true
• Nix cache restore — wires in nix-community/cache-nix-action@v7 using the same primary key and fallback prefix pattern (nix-Linux-nix-test--) as the main CI job, so the warmed Nix store is reused rather than rebuilt
• Restore-only (save: false) — setup steps consume the CI cache but never write their own entry, keeping cache management solely in CI
• Copilot instructions — adds .github/copilot-instructions.md that links to AGENTS.md for full development guidance and documents the Nix-specific requirements:
  • First-time setup: nix develop -c make bootstrap (required once on a fresh checkout before any other commands)
  • All common build/test/format commands shown with the nix develop -c prefix

[Alizter]

@copilot The PR description isn't very good. Please update it to mention the entire change rather than just the final one.

[Copilot]

@copilot The PR description isn't very good. Please update it to mention the entire change rather than just the final one.

Updated the PR description to cover all three commits:

1. Create .github/workflows/copilot-setup-steps.yml — installs Nix and enters the dev shell via nix develop -c true
2. Add Nix cache restore — wires in nix-community/cache-nix-action@v7 with the same key/prefix pattern as the main CI job so the warmed store is reused
3. Disable cache saving (save: false) — setup steps only fetch the existing cache, never write a new entry

[Alizter]

@copilot This doesn't enter any shell here

[Copilot]

Fixed in 3a271e4. The step now runs nix develop -c bash -c 'echo "$PATH"' | tr ':' '\n' >> "$GITHUB_PATH", which exports all the dev shell's PATH entries (the Nix store paths containing OCaml, opam, etc.) into $GITHUB_PATH so they're available to the Copilot agent in subsequent steps.

[Alizter]

@copilot No, I suppose you can leave it as -c true for now. Later when copilot is running commands it will need to remember to use nix develop -c.

[Alizter]

@copilot please come up with a plan on adding copilot instructions. It should probably have commands based on nix. Read AGENTS.md for an idea of what you should go for. If possible link directly to that file, but recall that when you first start up your dev environment you will need to call nix develop -c make bootstrap at least once.

[Copilot]

@copilot please come up with a plan on adding copilot instructions. It should probably have commands based on nix. Read AGENTS.md for an idea of what you should go for. If possible link directly to th...

Added .github/copilot-instructions.md in 49da9f5. It links to AGENTS.md for the full reference and covers:

• First-time setup: nix develop -c make bootstrap (needed once on a fresh checkout before anything else)
• All common commands from AGENTS.md shown with the nix develop -c prefix (build, test, fmt, promote, etc.)