Wire ChatGateway to ConversationsRepo so every user message is saved on
receipt (M1-001) and every assistant response is saved on agent:end with
accumulated streaming text (M1-002). Metadata includes model, provider,
tokenUsage (input/output/cache counts), toolCalls array, and timestamp
on each message record (M1-003). Auto-creates the conversation row when
the agent session is new.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>