Function add_to_managed_section 

pub fn add_to_managed_section(
    source: &Utf8Path,
    plaintext_abs_path: &Utf8Path,
) -> Result<()>

Additively merge plaintext_abs_path into yui’s managed .gitignore section, preserving every other entry already there. Used by yui secret encrypt to close the window where a freshly written plaintext sibling would be visible to git add until the next apply rewrites the full managed block (issue #71).

Unlike write_managed_section, this is non-destructive: it parses the existing section, unions the single new entry, sorts and dedupes, and rewrites only the block between the markers. Content above and below the markers is untouched.