Collapse the contextual-user detection APIs into a single ContextualUserFragment trait with default marker-based matching and wrapping. Update the fragment docs and registry naming to match the new blessed path, and fix the stale arc monitor test reference exposed by the refactor.
Co-authored-by: Codex <noreply@openai.com>
Reapply the standardized model-visible context fragment work on top of the latest origin/main as a clean squashed restack.
Co-authored-by: Codex <noreply@openai.com>
