Function fork_source_head 

pub(crate) fn fork_source_head(
    store: &Store,
    fs: &FileStore,
    agent_uid: &str,
) -> Option<Head>

The chain head of the agent agent_uid was forked from (its definition has a branch_label and a parent_id): what a fork with no conversation of its own starts from (ResumeGate::ForkFrom). Shared by the spawn’s gate and the pane’s resume preflight.