Document `Started` as the eager sender-side payload check and
`Yield` plus `Return` as receiver-side decoding boundaries without
promising a symmetric error relay.
Separate the working task-scoped codec encoder from the private
per-dialog decoder and the incomplete `@context` hook params.
Link the planned typed `Start` contract and sender-side argument
validation follow-up in #514.
(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))
Gud Boi
a40fb2ebde