-- Agent Workbench conversation history.
--
-- Adds a `transcript` column holding the full multi-turn message history of an
-- agent session as a JSON array (the SDK's `Vec<Message>`, serde-serialized).
-- Each `/api/agent/chat` round-trip loads this transcript into the agent before
-- the turn and writes the updated transcript back when the turn completes, so a
-- session resumed by id sees everything that happened in prior turns. NULL/empty
-- means a fresh session with no history yet.
ALTER TABLE agent_sessions ADD COLUMN transcript TEXT;