Context

PR #1966 (EPIC-02) grouped most Agent fields into named sub-structs. However, rate_limiter, focus, sidequest, and feedback_detector remain as loose fields on Agent<C>.

Found by: impl-critic (SUG-03) and reviewer (SUG-03)

Acceptance criteria

• All remaining loose non-core fields grouped into appropriate sub-structs
• Consider CompressionState for focus/sidequest (they are context-compression related)
• No public API changes