Prevent premature drafter structured output

[aheritier]

Summary

• switch the PR-review drafter to tool-mode structured output so mid-analysis narration cannot be treated as its terminal result
• upgrade the agent config to v16 and document the terminal __structured_output__ contract
• add YAML contract coverage and correct reusable-workflow permission guidance

Why

Native structured output is applied to every Anthropic completion. A drafter progress message can therefore be schema-shaped and returned as a final empty/incomplete draft, as observed in docker/sailor#2325. Tool mode ends the drafter only on its explicit, schema-valid output-tool call.

Validation

• focused PR-review agent YAML tests (71 passed)
• pnpm test (1,054 passed)
• pnpm lint
• docker-agent v1.140.0 dry-run config validation

No security, authorization, fail-closed, marker, or retry safeguards were weakened.