Pass stream as top-level request key in dream config
Stream was the only routing parameter (day, segment, facet, stream)
not passed as a top-level request key — it went only through env vars.
This caused config.get("stream") to return None in hooks after the
request→config merge, since env vars aren't merged into config keys.
Add request_config["stream"] = stream at all three dream spawn sites,
matching the existing pattern for day/segment/facet. The env var
fallback added in the prior commit (8fe14d90) remains as a safety net.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>