[P0][Tier 3] Add PR-size/checkpoint guidance for scoped, reviewable diffs

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Tier 3 — Workflow adoption · Priority: P0

Gap

No explicit guidance keeps agent-generated diffs scoped and reviewable (PR size, checkpoints, incremental commits).

Investment

• Add PR-size guidance, checkpointing, and incremental-commit practice to agent instructions.

Acceptance criteria

• Agent instructions include PR-size/checkpoint guidance.
• Generated diffs are scoped and reviewable in practice.

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Part of epic #3273