R2-3.1: group Agent struct fields into inner structs

**Epic**: #282 | **Phase**: 3 | **Priority**: P1 | **Size**: M

## Problem

Agent struct has 30+ flat fields. Builder has 14 methods. Hard to reason about state.

## Solution

Group into `AgentMemoryState<P>` and `AgentSkillState` inner structs. Non-breaking internal refactor — builder API unchanged.

**Blocked by**: #287 (further mod.rs decomposition first)