- page-triage classifier + light-extraction now put the static skill
prompt in `system:` so the per-document caches hit instead of
re-sending boilerplate in the user message every call.
- Description generation builders return `{ system, user }` with
instruction text + word limit moved into the cacheable system.
- Relationship-LLM proposal framing moved to `system:`.
- `KtxMessageBuilder.wrapSimple` skips the history breakpoint for
single-message calls (cache write that could never be reused).
- Gateway backend now sets `anthropic-beta: extended-cache-ttl-2025-04-11`
so 1h TTLs don't silently downgrade to 5m on Gateway routes.
