Linear stage chain.
N agents run in order. The incoming prompt hits stage 1. Each stage's response text becomes the prompt for the next stage. The last stage's response is the reply returned to the caller.
Options
:stages(required) -- a list of{name, module, opts}specs, one per stage, in order. Must be non-empty.
Concurrency
At most one prompt is in-flight at a time. Additional tell/ask
calls queue FIFO and begin when the current pipeline completes.
Failure semantics
A stage's turn error aborts the pipeline: the token fails with
{stage_name, reason} and the next queued prompt (if any) starts.
Stage process death halts the session (the chain is unrepairable).