* fix(nginx): defer cors to gateway allowlist
Remove proxy-level wildcard CORS handling so browser origins are controlled by the Gateway allowlist and stay aligned with CSRF origin checks.
* docs: document gateway cors allowlist
Clarify that same-origin nginx access needs no CORS headers while split-origin or port-forwarded browser clients must opt in with GATEWAY_CORS_ORIGINS.
* docs(gateway): record cors source of truth
Document that Gateway CORSMiddleware and CSRFMiddleware share GATEWAY_CORS_ORIGINS as the split-origin source of truth.
* fix(gateway): align cors origin normalization
* docs: clarify gateway langgraph routing
* docs(gateway): update runtime routing note