--schema t:people for loading the schema from a template

[simonw]

While working on the tutorial:

• Dedicated schemas page of documentation including a tutorial #788

I realized that it's hard to use llm logs --schema ... with a schema that was persisted in a template using --save.

I'm going to add a --schema t:name-of-template shortcut to resolve that.

[simonw]

Documented here: https://github.com/simonw/llm/blob/1bebf8b34ab7479c49ce429562a5cf6cd63fabdd/docs/schemas.md#how-to-specify-a-schema