Function stamp 

pub fn stamp(
    content: &str,
    id: &str,
    project: &str,
    worktree: &str,
    flow: &str,
) -> Result<Option<String>, FrontmatterError>

Writes id, project, worktree, and flow into the frontmatter without disturbing any other byte of the file. Returns None when the file already carries all four values, so callers can skip the write entirely.

Callers must resolve conflicts first: stamping never overwrites an existing id, project, or flow value.