Add origin/main sync to agent-ready startup

[dgalarza]

Summary

• Add an origin/main fetch + rebase step to the agent-ready AGENTS.md template Session Startup checklist
• Update agent-ready generation/audit guidance to include the same startup behavior
• Bump agent-ready to 1.3.1 and document the change in the changelog

Validation

• python3 .agents/skills/skill-creator/scripts/quick_validate.py plugins/agent-ready/skills/agent-ready
• python3 -m json.tool plugins/agent-ready/.claude-plugin/plugin.json >/dev/null
• python3 -m json.tool .claude-plugin/marketplace.json >/dev/null