fn read_archive_preview(filestore: &FileStore, zone: &str) -> (String, usize)Expand description
Pull a small preview + node_count out of an archive’s
output.state.json. Returns ("", 0) on any error.
Mirrors the heuristics used by read_session_preview in
agent_handlers.rs (skip the bootstrap “# Session Context” message
when a later user_message exists; cap at 240 chars).