Prompt template builder

Rendering is plain text substitution: no model is called and no tokens are counted, so judge the output size yourself or pair this with the tokenizer. The variables JSON doubles as the template's schema, and the test-case JSON doubles as its regression suite: a template change that breaks a case shows up here as a missing value or a type error. The template JSON export is the schema plus template text for version control, and the JSONL export feeds evaluation runs one prompt per line.