code-review-contextlisted

Codex maintains a context (history of messages) that is sent to the model in inference requests. 1. No history rewrite - the context must be built up incrementally. 2. Avoid frequent changes to context that cause cache misses. 3. No unbounded items - everything injected in the model context must have a bounded size and a hard cap. 4. No items larger than 10K tokens. 5. Highlight new individual items that can cross >1k tokens as P0. These need an additional manual review. 6. All injected fragments must be defined as structs in `core/context` and implement ContextualUserFragment trait