Follow up to: #3582504 _**NOTE: Before working on this issue, verify it's still relevant and makes sense.**_ **Description:** Some future features need entity fields that are easiest to reserve before stable schema settles. Adding the fields now avoids extra update hooks later while introducing little or no UI or behavior change. This issue covers schema-only reservations from the CCC context engineering recommendations, plus related pre-RC1 schema reservations referenced by the RC1 impact analysis. **Scope:** - Add `review_by` nullable datetime field to `AiContextItem`. - Add `auto_expire` boolean field to `AiContextItem`, default `FALSE`. - Add `selection_reasoning` nullable serialized/map field to `AiContextUsage`. - Confirm and, if still needed, add `content_format` field to `AiContextItem`. - Confirm and, if still needed, add `weight` field to `AiContextItem`. - Add required update hooks/entity definition updates for existing installs. - Do not implement staleness filtering, cron expiration, audit UI, reasoning instrumentation, custom formatting behavior, or explicit ordering behavior in this issue. **Acceptance criteria:** - New installs have the reserved fields. - Existing installs receive the fields through update hooks. - Reserved fields have clear defaults and do not change selection/render behavior unless explicitly required. - Fields are not exposed in user-facing UI unless already required by entity display defaults. - Tests or update-path coverage verify fields exist after install/update. **AI usage disclosures:** - [x] AI Assisted Issue - [x] AI Generated Code