Stage authoring guidance: each stage ends at a logical commit

Authoring docs: the per-stage commit contract plus "how to split an agent into stages" guidance, from the call's own lesson (the 2-stage CVE example). Each stage should end at a logical commit so stages compose, resume, and review cleanly.

Done when:
- The authoring guide documents the per-stage commit contract and the stage-splitting guidance.

Depends on: none — pairs with #122 (entry-point docs).

Locate the authoring guide and review the call's 2-stage CVE example for the intended stage-splitting guidance. Document the per-stage commit contract and explain how to split an agent into stages; it is done when both topics are covered and stages are described as composing, resuming, and reviewing cleanly.

Written by the indexing model from the issue text.