Function handle 

pub fn handle(
    root: &Path,
    ticket_dir: &Path,
    store: &Store,
    args: &PostArgs,
    now_ms: i64,
    ticket_prefix: &str,
    agent: Option<&AgentConfig>,
    flows: &BTreeMap<String, Flow>,
    default_flow: &str,
) -> Result<Value, PostError>

Registers a ticket file: validates and stamps frontmatter, indexes the ticket, and for auto or at creates one queued activation. Reposting a stamped file is idempotent. The dispatcher is the only caller, so plain reads before writes here cannot race another writer.