[M0] Expand the pull request governance template

[arthurpanhku]

Goal

Make intent, risk, evidence, and recovery information reviewable before a change is merged.

Scope

Expand the pull request template with required Goal, Scope, Plan, Success Criteria, Risk/Mitigations, Evidence, and Rollback/Escalation sections.

Acceptance criteria

• Each required section is present with concise author guidance.
• The template works for both human-authored and agent-authored changes.
• Authors can explicitly mark a section not applicable and explain why.
• The Plan Gate can parse the required headings reliably.

Dependencies

Coordinate heading names and allowed placeholders with the Plan Gate issue.