D715 — concept_new template variables accept lists, only as whole values

Decision. concept_new vars values are a string or a list of strings. A list fills a frontmatter value that is exactly {{name}} (the key becomes a list) or is spliced in place of a list item that is exactly {{name}}. A list used anywhere else (inside a larger string, in a list item with other text, in the body) is an error and nothing is written. The caller's value decides the kind; templates declare nothing.

Why. With string-only vars, roles: [{{roles}}] plus "a, b" yields one quoted item, so a multi-item list could not come from a variable. Whole-value positions are the only ones where a list has a single unambiguous YAML rendering.

Alternatives rejected. Joining a list into text with a separator: invents a separator and hides data loss. Auto-splitting strings on commas: changes today's output for string-only calls. A per-variable kind in template_list: extra surface for a choice the caller already makes.

Consequences. String-only calls render byte-identically. List items are inserted literally like strings. Any new template position that renders a variable must go through the same whole-value check.