Temporal message identity, artifact exclusion, and user-prose retention

Derive temporal_messages.id from tenant, project, session, and source rather than caller source_id; sync, vectors, distillation links, and multiple tenants require stable collision-free keys. Never expose synthetic resolution or refcheck artifacts to the model or persist them in temporal storage; they are internal probes, and the resolver must recover internally rather than throw into the request path. During oversized-message reduction, retain the leading user-prose prefix verbatim (userBlobHeadChars, default 1500 and configurable to 0) before relevance scoring. Relevance-only reduction can discard opening constraints while retaining incidental later text. Elided bulk remains searchable through temporal_messages and FTS.