[Copilot]

Add useful unit tests for core astronomical conversions and calculations, configure CI to install and run testthat, and document package testing and contribution conventions.

• Tests: Cover coordinate conversions, spherical transforms, sky area, coordinate matching, and cosmological distances, including validation and boundary cases.
• Package and CI: Add testthat to Suggests, select edition 3, and ensure CI installs check dependencies.
• Contributor guidance: Add AGENTS.md with package structure, test commands, and change conventions.

[Copilot]

🔵 Needs a closer look

The RA=0 round-trip assertion may wrap to 360, so its 1e-10 tolerance needs validation across the R/testthat CI matrix because R was unavailable for runtime verification.

No open issues 🎉

What changed in this PR

Adds automated test coverage for celestial’s core astronomical utilities, integrates testthat into package checks, and documents contributor conventions.

Changes:

• Add deterministic tests for coordinate conversions, matching, spherical transforms, sky area, and cosmological distances.
• Configure testthat edition 3 and CI check dependencies.
• Document package structure, testing commands, and contribution guidance.

File	Description
tests/​testthat/​test-spherical-coordinates.R	Test spherical/Cartesian and equatorial/galactic round trips.
tests/​testthat/​test-sky-area.R	Test full-sky area, unit conversions, and invalid inputs.
tests/​testthat/​test-cosmology.R	Test distance identities, ordering, and redshift validation.
tests/​testthat/​test-coordinate-matching.R	Test matching, separation units, nearest sources, and linked groups.
tests/​testthat/​test-coordinate-conversions.R	Test degree/DMS/HMS conversions and domain validation.
tests/​testthat.R	Add the package testthat entry point.
DESCRIPTION	Declare testthat and select edition 3.
AGENTS.md	Document testing and contribution conventions.
.github/​workflows/​main.yml	Configure CI to install testthat and check dependencies.

_{Review effort: Balanced}