Cross-template dependencies

### TL;DR

We'd like a template to be able to "include" another template, as a kind of subroutine.

TODO describe more

cc @capri-xiyue

The issue mentions cross-template dependencies and includes, but the TODO indicates the description is incomplete. Look at the template system in the codebase, likely under a 'templates' or 'pkg/template' directory. Understand how templates are currently parsed and rendered. Determine what a 'subroutine' inclusion would entail, such as parsing dependencies, handling variable scopes, and avoiding cycles. Check for existing issues or PRs about template features.

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