Server-side webhook agent runs (sync, async, and admin test) used Stream:false,
so OpenAI-compatible routers that only cache streaming requests never populated
or served their prompt cache. Webhook runs paid full input-token price on every
turn even with a stable session and an identical multi-turn prefix, while WS chat
(Stream:true) got cache hits from the 3rd turn.
Add gateway.webhook_stream (default true, env GOCLAW_WEBHOOK_STREAM) and apply it
to all three run sites. ChatStream returns the fully assembled response, so the
payload returned to the caller is unchanged. Set to false to restore non-streaming.
