Function compose_default 

pub fn compose_default(
    store: &Store,
    type_: &str,
    frontmatter: &Frontmatter,
    body: &str,
) -> Result<String>

Compose a deterministic default summary for a file from its type, frontmatter, and body. If the store’s ## Schemas declares a summary_template for the type, it is rendered with {field} interpolation; otherwise the default is the body’s first non-heading paragraph. The result is always single-line and ≤ MAX_SUMMARY_LEN chars.

The tool generates a deterministic floor; the agent provides the ceiling via dbmd fm set <file> summary='…'.