Global template cache folder + envvar to control its location

[wukevin]

Description

Redirects template structures to be stored in a globally-shared folder (default repo_root / downloads / template_cifs) that can be overridden by a environment variable CHAI_TEMPLATE_CIF_FOLDER.

Motivation

Inspired by #370 - allows re-using template structures that have already been downloaded, also custom structures (though these need to be saved in the global cache folder as identifier.cif.gz.

Test plan

Tested locally; verified that cache exists and is reused between runs.

[Copilot]

Pull Request Overview

This PR redirects the storage of template structures to a globally shared folder, allowing reuse of cached templates across runs and custom overrides with an environment variable.

• Updated the docstring for downloading CIF files to indicate no-op behavior when file exists.
• Introduced a global constant for the template CIF cache folder with an environment variable override.
• Removed the local template cache folder parameter in favor of the global setting.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
chai_lab/data/io/rcsb.py	Updated CIF file download docstring to detail no-op behavior when file exists.
chai_lab/data/dataset/templates/context.py	Added global TEMPLATE_CIF_FOLDER from environment variable with a default value.
chai_lab/chai1.py	Removed passing of a local template cache folder to use the global setting instead.