Function document_active_identities 

pub fn document_active_identities(
    content: &str,
) -> BTreeMap<String, Vec<String>>

Build the active document identity registry for tracked-work lookup.

Each normalized #id maps to the active sources that define it: frontmatter prompt_presets, active backlog/review/icebox items, or both. Done items and agent:done archives are intentionally excluded because they are not active dispatch targets.