Template ergonomics and power

[httpdss]

Summary
Expand built-in Jinja filters/globals and improve documentation of custom delimiters.

Proposed changes

• Add filters/globals: uuid(), now(), env(name), read_file(path), to_yaml/from_yaml, to_json/from_json.
• Keep existing filters: slugify, latest_release, default_branch.
• Document custom delimiters ({%@ @%}, {{@ @}}) prominently with examples.

Files

• struct_module/template_renderer.py
• docs/template-variables.md

Acceptance criteria

• New helpers available in templates and documented with examples.
• Tests/smoke templates exercising new helpers.