Add GitHub composite action for installing Agda and standard library

[Copilot]

Implements a reusable composite action that installs Agda from official releases with optional standard library support.

Implementation

• Platform detection: Automatically selects correct binary for Linux, Windows, macOS ARM64, and macOS x64
• Inputs: agda-version (required), agda-stdlib-version (optional)
• Outputs: agda-path, agda-dir
• Installation flow: Downloads official release binary → extracts → installs to ~/.local/bin → configures Agda directories → optionally installs stdlib

Usage

- name: Setup Agda
  uses: agda/agda-setup-action@v1
  with:
    agda-version: '2.8.0'
    agda-stdlib-version: '2.3'  # optional

- name: Compile Agda code
  run: agda Main.agda

Files

• action.yml: Composite action with conditional stdlib installation
• README.md: Documentation with examples
• .github/workflows/test.yml: Multi-platform validation (Ubuntu, Windows, macOS ARM/Intel)
• .github/workflows/example.yml: Usage demonstration with stdlib imports

Binary naming patterns verified against Agda 2.8.0 release: Agda-v{version}-{linux|win64|macOS-arm64|macOS-x64}.{tar.xz|zip}

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:

• https://api.github.com/repos/agda/agda/releases
  • Triggering command: curl -s REDACTED (http block)
• https://api.github.com/repos/agda/agda/releases/latest
  • Triggering command: curl -s REDACTED (http 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

A github composite action that installs Agda from releases at #agda/agda, and possibly the standard library. Input parameters to the action should be the version of Agda and the version of the std-lib. If no std-lib version is given, don't install the std-lib. Outputs: the path to the agda executable and the agda-dir. Usable under images ubuntu/windows/macos-latest & macos-15-intel. Similar to https://github.com/graded-type-theory/graded-type-theory/blob/master/.github/workflows/deploy.yml

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[andreasabel]

Great job!

Please address the inline comments, these are minor issues.

[andreasabel]

Restrict these two imports to the .Base submodules.

[andreasabel]

Use v5 of the checkout action, here and everywhere.

[andreasabel]

Refactor this so that there is only one test job but running in a matrix of the four images.

[andreasabel]

End each of the 4 main bullet points here with a ".".