Add checkout-as-app action and enhance github-app-auth

[nsheaps]

Summary

This PR introduces a new GitHub Action (checkout-as-app) that combines GitHub App authentication with repository checkout, and enhances the existing github-app-auth action with additional functionality.

Key Changes

New Action: checkout-as-app

• Created a new composite action that authenticates as a GitHub App and checks out a repository with the app token baked into the remote URL
• Enables subsequent git operations to automatically use the app credentials
• Passes through all standard actions/checkout inputs (repository, ref, path, clean, filter, sparse-checkout, fetch-depth, fetch-tags, lfs, submodules, etc.)
• Outputs both authentication info (token, app-slug, user-id, user-name, user-email) and checkout info (ref, commit)

Enhanced github-app-auth Action

• Added skip-checkout input to allow the action to be used without performing its own checkout (useful when called from checkout-as-app)
• Added user-email output that constructs the GitHub App bot email in the standard format: {user-id}+{user-name}@users.noreply.github.com
• Refactored user info retrieval to compute email address alongside user ID and name
• Added informational logging to output GitHub App authentication details
• Made the built-in checkout step conditional based on the skip-checkout input
• Updated git config commands to use the new user-email output instead of inline computation

Implementation Details

• The checkout-as-app action is a two-step composite action: first it authenticates via github-app-auth with skip-checkout: true, then it performs checkout using actions/checkout with the obtained token
• Email address generation follows GitHub's standard format for bot accounts
• All inputs are properly documented with descriptions and defaults
• The action maintains backward compatibility with existing github-app-auth usage

https://claude.ai/code/session_013AzQ3VqAJ2sGSyUFDRBS2X