docs/kong.md: a plain route or ai-proxy with llmgw as the upstream, the
client's llmgw key passed on so per-team budgets hold, Kong as a provider
behind llmgw, and Kong 3.9's streamed answers counted without completion
tokens. Closes BACKLOG L9.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>