refactor(contexto): Update episodic payload

[DaevMithran]

Summary

• buildEpisodePayload no longer splits messages into userMessage / assistantMessages / toolMessages. The full messages array is now sent as-is under data.messages.
• stopReason is removed from the episode context block — it was unused downstream.
• estimatePayloadTokens updated to read from data.messages.
• Role-based separation moves to the backend, which is the source of truth for how episodes are interpreted and stored.

Why

Splitting messages by role in the client coupled the SDK to one backend's data model. Sending the raw turn lets the backend decide how to slice it, keeps the client a thin passthrough, and removes a small but real surface area for shape drift between client and server.