Add lightweight contribution cleanup guidance

[provencher]

Summary

• add a brief, non-blocking final cleanup pass to the contribution-check skill
• prompt agents to minimize maintenance surface, low-value tests, and scope drift without targeting raw line count
• surface the guidance in agent instructions and push-preflight output

Validation

• .agents/skills/rpce-contribution-check/scripts/preflight.sh commit
• .agents/skills/rpce-contribution-check/scripts/preflight.sh push
• shell syntax, whitespace, skill frontmatter, and metadata checks