modernize docs, tests, with a ci flow

[eschultink]

• modernize everything, CI tests, etc.

Change implications

• includes or requires infrastructure changes? No infrastructure changes are included or required in this pull request.

[Copilot]

Pull request overview

This pull request modernizes the codebase by updating tests, documentation, and CI infrastructure. The changes improve error handling in the validateFrequency function by making it throw errors instead of just logging, add comprehensive test coverage for this behavior, enhance documentation with usage guidance, and modernize the GitHub Actions workflow with multi-version Node.js testing.

Changes:

• Enhanced validateFrequency to throw errors when invalid frequencies are provided, with corresponding test coverage
• Added AGENTS.md documentation file explaining repository structure and purpose for AI agents
• Modernized GitHub Actions CI workflow with newer action versions (v4), Node.js matrix testing (20.x, 22.x, 24.x, 25.x), and additional workflow triggers
• Updated README with guidance on when to use this tool versus native Firestore scheduled backups
• Applied consistent code formatting throughout test files

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/cmd.js	Added test coverage for validateFrequency function's error throwing behavior and applied consistent code formatting
lib/cmd.js	Enhanced validateFrequency to throw errors instead of only logging to console, improving error handling
README.md	Added note clarifying this tool is for backing up subset of entities, not entire databases
AGENTS.md	New documentation file explaining repository structure, purpose, and technical details for AI agents
.github/workflows/test.yml	Modernized CI with actions v4, Node.js version matrix (20.x-25.x), and additional triggers (pull_request, workflow_dispatch)

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