docs: clarify PR completion invariant - #7339

Ladon verdict: Escalate to human review

Escalate — gated path requires human/CODEOWNERS review.

This is a documentation-only PR (12 additions) adding a "completion invariant" as item 6 of the PR Preparation Checklist in .agents/playbook.md plus a consistent pointer stub in CLAUDE.md. The reviewer found no Critical/High/Medium findings — content is coherent, list numbering is correct, and no protocol surface is touched.

However, .agents/playbook.md matches the gated glob .agents/** (gated_paths: true) and review_decision is REVIEW_REQUIRED, not APPROVED. Per decision rule row 2, the hard approval gate is not satisfied, so this cannot auto-approve regardless of the clean diff. Human/CODEOWNERS review of .agents/playbook.md is required to lift the gate.

Escalation reasons

• .agents/playbook.md is under a hard approval gate (.agents/**) and review_decision is REVIEW_REQUIRED — human/CODEOWNERS approval required before merge.

Why human review

• .agents/playbook.md (modified) matches gated path .agents/** and review_decision is REVIEW_REQUIRED — human/CODEOWNERS approval required to satisfy the hard approval gate
• This PR touches a path under a hard, non-overridable approval gate (.agents/playbook.md (modified) matches .agents/**) and the current GitHub review decision is 'REVIEW_REQUIRED', not APPROVED. This is a hard gate enforced in code — Ladon cannot auto-approve until a human/CODEOWNERS approval is recorded, regardless of how clean the diff is.