[codex] clarify agent subject preparation

[8Dionysus]

What changed

• add a dedicated subject-preparation surface and route the agent docs toward it
• align agent model and profile docs with the new subject-prep posture

Why

• preparation groundwork for Agon needs the agent layer to name its preparatory posture explicitly instead of leaving it implied across several docs

Validation

• docs change reviewed in the current repo context
• git diff --check