The 1630-line start.rs is split into a sub-module directory:
- validation.rs: validate_agent_stage + read_front_matter_agent helpers (69 lines)
- spawn.rs: run_agent_spawn — the background async work that was inlined as
a tokio::spawn closure body inside start_agent (359 lines)
- mod.rs: AgentPool::start_agent orchestrator + tests (1062 lines)
Stage validation and front-matter agent reading are pre-lock pure helpers that
naturally extract. The spawn closure body becomes a free async fn that takes
the previously-cloned values as parameters; rebound to the original _clone /
_owned names at the top of the body so the actual work code is a verbatim copy.
No behaviour change. All 23 start tests pass; full suite green.
dave